Nox system test refactor (#60)

* Add basic noxfile to orchestrate system tests
* Move explicit system tests to nox
* Add cloud sdk environment automation
diff --git a/system_tests/nox.py b/system_tests/nox.py
new file mode 100644
index 0000000..67f9900
--- /dev/null
+++ b/system_tests/nox.py
@@ -0,0 +1,211 @@
+# Copyright 2016 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Noxfile for automating system tests.
+
+This file handles setting up environments needed by the system tests. This
+separates the tests from their environment configuration.
+
+See the `nox docs`_ for details on how this file works:
+
+.. _nox docs: http://nox.readthedocs.io/en/latest/
+"""
+
+import os
+
+from nox.command import which
+import py.path
+
+
+HERE = os.path.dirname(__file__)
+DATA_DIR = os.path.join(HERE, 'data')
+SERVICE_ACCOUNT_FILE = os.path.join(DATA_DIR, 'service_account.json')
+AUTHORIZED_USER_FILE = os.path.join(DATA_DIR, 'authorized_user.json')
+EXPLICIT_CREDENTIALS_ENV = 'GOOGLE_APPLICATION_CREDENTIALS'
+EXPLICIT_PROJECT_ENV = 'GOOGLE_CLOUD_PROJECT'
+EXPECT_PROJECT_ENV = 'EXPECT_PROJECT_ID'
+
+# The download location for the Cloud SDK
+CLOUD_SDK_DIST_FILENAME = 'google-cloud-sdk.tar.gz'
+CLOUD_SDK_DOWNLOAD_URL = (
+    'https://dl.google.com/dl/cloudsdk/release/{}'.format(
+        CLOUD_SDK_DIST_FILENAME))
+
+# This environment variable is recognized by the Cloud SDK and overrides
+# the location of the SDK's configuration files (which is usually at
+# ${HOME}/.config).
+CLOUD_SDK_CONFIG_ENV = 'CLOUDSDK_CONFIG'
+
+# If set, this is where the environment setup will install the Cloud SDK.
+# If unset, it will download the SDK to a temporary directory.
+CLOUD_SDK_ROOT = os.environ.get('CLOUD_SDK_ROOT')
+
+if CLOUD_SDK_ROOT is not None:
+    CLOUD_SDK_ROOT = py.path.local(CLOUD_SDK_ROOT)
+    CLOUD_SDK_ROOT.ensure(dir=True)  # Makes sure the directory exists.
+else:
+    CLOUD_SDK_ROOT = py.path.local.mkdtemp()
+
+# The full path the cloud sdk install directory
+CLOUD_SDK_INSTALL_DIR = CLOUD_SDK_ROOT.join('google-cloud-sdk')
+
+# The full path to the gcloud cli executable.
+GCLOUD = str(CLOUD_SDK_INSTALL_DIR.join('bin', 'gcloud'))
+
+# gcloud requires Python 2 and doesn't work on 3, so we need to tell it
+# where to find 2 when we're running in a 3 environment.
+CLOUD_SDK_PYTHON_ENV = 'CLOUDSDK_PYTHON'
+CLOUD_SDK_PYTHON = which('python2', None)
+
+# Cloud SDK helpers
+
+
+def install_cloud_sdk(session):
+    """Downloads and installs the Google Cloud SDK."""
+    # Configure environment variables needed by the SDK.
+    # This sets the config root to the tests' config root. This prevents
+    # our tests from clobbering a developer's configuration when running
+    # these tests locally.
+    session.env[CLOUD_SDK_CONFIG_ENV] = str(CLOUD_SDK_ROOT)
+    # This tells gcloud which Python interpreter to use (always use 2.7)
+    session.env[CLOUD_SDK_PYTHON_ENV] = CLOUD_SDK_PYTHON
+
+    # If the glcoud already exists, we don't need to do anything else.
+    # Note that because of this we do not attempt to update the sdk -
+    # if the CLOUD_SDK_ROOT is cached, it will need to be periodically cleared.
+    if py.path.local(GCLOUD).exists():
+        return
+
+    tar_path = CLOUD_SDK_ROOT.join(CLOUD_SDK_DIST_FILENAME)
+
+    # Download the release.
+    session.run(
+        'wget', CLOUD_SDK_DOWNLOAD_URL, '-O', str(tar_path), silent=True)
+
+    # Extract the release.
+    session.run(
+        'tar', 'xzf', str(tar_path), '-C', str(CLOUD_SDK_ROOT))
+    session.run(tar_path.remove)
+
+    # Run the install script.
+    session.run(
+        str(CLOUD_SDK_INSTALL_DIR.join('install.sh')),
+        '--usage-reporting', 'false',
+        '--path-update', 'false',
+        '--command-completion', 'false',
+        silent=True)
+
+
+def copy_credentials(credentials_path):
+    """Copies credentials into the SDK root as the application default
+    credentials."""
+    dest = CLOUD_SDK_ROOT.join('application_default_credentials.json')
+    if dest.exists():
+        dest.remove()
+    py.path.local(credentials_path).copy(dest)
+
+
+def configure_cloud_sdk(
+        session, application_default_credentials, project=False):
+    """Installs and configures the Cloud SDK with the given application default
+    credentials.
+
+    If project is True, then a project will be set in the active config.
+    If it is false, this will ensure no project is set.
+    """
+    install_cloud_sdk(session)
+
+    if project:
+        session.run(GCLOUD, 'config', 'set', 'project', 'example-project')
+    else:
+        session.run(GCLOUD, 'config', 'unset', 'project')
+
+    # Copy the credentials file to the config root. This is needed because
+    # unfortunately gcloud doesn't provide a clean way to tell it to use
+    # a particular set of credentials. However, this does verify that gcloud
+    # also considers the credentials valid by calling application-default
+    # print-access-token
+    session.run(copy_credentials, application_default_credentials)
+
+    # Calling this forces the Cloud SDK to read the credentials we just wrote
+    # and obtain a new access token with those credentials. This validates
+    # that our credentials matches the format expected by gcloud.
+    # Silent is set to True to prevent leaking secrets in test logs.
+    session.run(
+        GCLOUD, 'auth', 'application-default', 'print-access-token',
+        silent=True)
+
+
+# Test sesssions
+
+
+def session_service_account(session):
+    session.virtualenv = False
+    session.run('pytest', 'test_service_account.py')
+
+
+def session_oauth2_credentials(session):
+    session.virtualenv = False
+    session.run('pytest', 'test_oauth2_credentials.py')
+
+
+def session_default_explicit_service_account(session):
+    session.virtualenv = False
+    session.env[EXPLICIT_CREDENTIALS_ENV] = SERVICE_ACCOUNT_FILE
+    session.env[EXPECT_PROJECT_ENV] = '1'
+    session.run('pytest', 'test_default.py')
+
+
+def session_default_explicit_authorized_user(session):
+    session.virtualenv = False
+    session.env[EXPLICIT_CREDENTIALS_ENV] = AUTHORIZED_USER_FILE
+    session.run('pytest', 'test_default.py')
+
+
+def session_default_explicit_authorized_user_explicit_project(session):
+    session.virtualenv = False
+    session.env[EXPLICIT_CREDENTIALS_ENV] = AUTHORIZED_USER_FILE
+    session.env[EXPLICIT_PROJECT_ENV] = 'example-project'
+    session.env[EXPECT_PROJECT_ENV] = '1'
+    session.run('pytest', 'test_default.py')
+
+
+def session_default_cloud_sdk_service_account(session):
+    session.virtualenv = False
+    configure_cloud_sdk(session, SERVICE_ACCOUNT_FILE)
+    session.env[EXPECT_PROJECT_ENV] = '1'
+    session.run('pytest', 'test_default.py')
+
+
+def session_default_cloud_sdk_authorized_user(session):
+    session.virtualenv = False
+    configure_cloud_sdk(session, AUTHORIZED_USER_FILE)
+    session.run('pytest', 'test_default.py')
+
+
+def session_default_cloud_sdk_authorized_user_configured_project(session):
+    session.virtualenv = False
+    configure_cloud_sdk(session, AUTHORIZED_USER_FILE, project=True)
+    session.env[EXPECT_PROJECT_ENV] = '1'
+    session.run('pytest', 'test_default.py')
+
+
+def session_compute_engine(session):
+    session.virtualenv = False
+    session.run('pytest', 'test_compute_engine.py')
+
+
+def session_app_engine(session):
+    session.virtualenv = False
+    session.run('pytest', 'app_engine/test_app_engine.py')