chore: omit 'docfx' from docs-presubmit build (#831)

We don't have a 'docfx' session in our noxfile (see #822).

Further tweak '.kokoro' configuration to get 'Kokoro docs-presubmit' running:

- Set up additional 'gfile_resource' in '.kokoro/docs/common.cfg'.
- Get 'gcloud' installed in '.kokoro/docker/docs/Dockerfile'
- Run 'docs' session with Python 3.8.

Closes #797.
diff --git a/.kokoro/docker/docs/Dockerfile b/.kokoro/docker/docs/Dockerfile
index 4e1b1fb..a6bcfc8 100644
--- a/.kokoro/docker/docs/Dockerfile
+++ b/.kokoro/docker/docs/Dockerfile
@@ -64,4 +64,12 @@
   && python3.8 /tmp/get-pip.py \
   && rm /tmp/get-pip.py
 
+# Install gcloud SDK
+RUN echo "deb [signed-by=/usr/share/keyrings/cloud.google.gpg] http://packages.cloud.google.com/apt cloud-sdk main" | \
+    tee -a /etc/apt/sources.list.d/google-cloud-sdk.list \
+  && curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | \
+    apt-key --keyring /usr/share/keyrings/cloud.google.gpg  add - \
+  && apt-get update -y \
+  && apt-get install python2 google-cloud-sdk -y
+
 CMD ["python3.8"]
diff --git a/.kokoro/docs/common.cfg b/.kokoro/docs/common.cfg
index 24c8c89..2118d7d 100644
--- a/.kokoro/docs/common.cfg
+++ b/.kokoro/docs/common.cfg
@@ -10,6 +10,9 @@
 # Download trampoline resources.
 gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/trampoline"
 
+# Download resources for system tests (service account key, etc.)
+gfile_resources: "/bigstore/cloud-devrel-kokoro-resources/google-auth-library-python"
+
 # Use the trampoline script to run in docker.
 build_file: "google-auth-library-python/.kokoro/trampoline_v2.sh"
 
@@ -62,4 +65,4 @@
       keyname: "docuploader_service_account"
     }
   }
-}
\ No newline at end of file
+}
diff --git a/.kokoro/docs/docs-presubmit.cfg b/.kokoro/docs/docs-presubmit.cfg
index d0f5783..d3f0dea 100644
--- a/.kokoro/docs/docs-presubmit.cfg
+++ b/.kokoro/docs/docs-presubmit.cfg
@@ -24,5 +24,5 @@
 # Only run this nox session.
 env_vars: {
     key: "NOX_SESSION"
-    value: "docs docfx"
+    value: "docs"
 }
diff --git a/noxfile.py b/noxfile.py
index d375b03..caeb272 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -142,7 +142,7 @@
     )
 
 
-@nox.session(python="3.7")
+@nox.session(python="3.8")
 def docs(session):
     """Build the docs for this library."""
 
diff --git a/owlbot.py b/owlbot.py
index f692f70..fa03d69 100644
--- a/owlbot.py
+++ b/owlbot.py
@@ -7,11 +7,39 @@
 # Add templated files
 # ----------------------------------------------------------------------------
 templated_files = common.py_library(unit_cov_level=100, cov_level=100)
+
+
 s.move(
     templated_files / ".kokoro",
     excludes=[
         "continuous/common.cfg",
+        "docs/common.cfg",
         "presubmit/common.cfg",
         "build.sh",
     ],
 )  # just move kokoro configs
+
+
+assert 1 == s.replace(
+    ".kokoro/docs/docs-presubmit.cfg",
+    'value: "docs docfx"',
+    'value: "docs"',
+)
+
+assert 1 == s.replace(
+    ".kokoro/docker/docs/Dockerfile",
+    """\
+CMD \["python3\.8"\]""",
+    """\
+# Install gcloud SDK
+RUN echo "deb [signed-by=/usr/share/keyrings/cloud.google.gpg] http://packages.cloud.google.com/apt cloud-sdk main" | \\
+    tee -a /etc/apt/sources.list.d/google-cloud-sdk.list \\
+  && curl https://packages.cloud.google.com/apt/doc/apt-key.gpg | \\
+    apt-key --keyring /usr/share/keyrings/cloud.google.gpg  add - \\
+  && apt-get update -y \\
+  && apt-get install python2 google-cloud-sdk -y
+
+CMD ["python3.8"]""",
+)
+
+s.shell.run(["nox", "-s", "blacken"], hide_output=False)