Use new Python image for Kokoro tests (GoogleCloudPlatform#1257)

Author: Jon Wayne Parrott

.kokoro/common.cfg

@@ -9,5 +9,5 @@ build_file: "python-docs-samples/.kokoro/trampoline.sh"
 # Use the Python worker docker iamge.
 env_vars: {
     key: "TRAMPOLINE_IMAGE"
-    value: "gcr.io/silver-python2/python-worker"
+    value: "gcr.io/cloud-devrel-kokoro-resources/python"
 }

.kokoro/system_tests.sh

@@ -27,9 +27,7 @@ export GOOGLE_APPLICATION_CREDENTIALS=$(pwd)/testing/service-account.json
 export GOOGLE_CLIENT_SECRETS=$(pwd)/testing/client-secrets.json
 
 # Run tests
-pip install --user --upgrade pip setuptools
-~/.local/bin/pip install --user --upgrade nox-automation
-~/.local/bin/nox -k "${NOX_SESSION}" || ret_code=$?
+nox -k "${NOX_SESSION}" || ret_code=$?
 
 # Workaround for Kokoro permissions issue: delete secrets
 rm testing/{test-env.sh,client-secrets.json,service-account.json}