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/tox.ini b/tox.ini
index 6864dde..427f2b7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -23,21 +23,27 @@
 
 [testenv:py35-system]
 basepython = python3.5
+changedir = {toxinidir}/system_tests
 commands =
-  py.test --ignore system_tests/app_engine/app {posargs:system_tests}
+  nox {posargs}
 deps =
   {[testenv]deps}
+  nox-automation
 passenv =
   SKIP_APP_ENGINE_SYSTEM_TEST
+  CLOUD_SDK_ROOT
 
 [testenv:py27-system]
 basepython = python2.7
+changedir = {toxinidir}/system_tests
 commands =
-  py.test --ignore system_tests/app_engine/app {posargs:system_tests}
+  nox {posargs}
 deps =
   {[testenv]deps}
+  nox-automation
 passenv =
   SKIP_APP_ENGINE_SYSTEM_TEST
+  CLOUD_SDK_ROOT
 
 [testenv:docgen]
 basepython = python3.5