testing: various cleanups

[tmatsuo]

• activate the secret service account only when the key is there
• merge automl_secrets.txt into test-env.sh
• if .kokoro/docker or .kokoro/tests have changes, test everything
• remove stale apt-get command
• use SECRET_MANAGER_PROJECT envvar for secret manager command,
  fallback to cloud-devrel-kokoro-resources

fixes #3820

[busunkim96]

Is the Bigquery failure a flake?

Test
natality_tutorial_test.test_natality_tutorial

Run time 0:00:00.473
Failed: google.api_core.exceptions.PreconditionFailed: 412 POST https://bigquery.googleapis.com/bigquery/v2/projects/python-docs-samples-tests/datasets: IAM setPolicy failed for Dataset python-docs-samples-tests:natality_regression_6f9938f5_60fe_47c0_9424_41d8d1b3db7a: There were concurrent policy changes. Please retry the whole read-modify-write with exponential backoff.
Traceback (most recent call last):
  File "/tmpfs/src/github/python-docs-samples/bigquery/cloud-client/natality_tutorial_test.py", line 42, in test_natality_tutorial
    natality_tutorial.run_natality_tutorial(override_values)
  File "/tmpfs/src/github/python-docs-samples/bigquery/cloud-client/natality_tutorial.py", line 50, in run_natality_tutorial
    dataset = client.create_dataset(dataset)
  File "/tmpfs/src/github/python-docs-samples/bigquery/cloud-client/.nox/py-3-7/lib/python3.7/site-packages/google/cloud/bigquery/client.py", line 461, in create_dataset
    retry, method="POST", path=path, data=data, timeout=timeout
  File "/tmpfs/src/github/python-docs-samples/bigquery/cloud-client/.nox/py-3-7/lib/python3.7/site-packages/google/cloud/bigquery/client.py", line 556, in _call_api
    return call()
  File "/tmpfs/src/github/python-docs-samples/bigquery/cloud-client/.nox/py-3-7/lib/python3.7/site-packages/google/api_core/retry.py", line 286, in retry_wrapped_func
    on_error=on_error,
  File "/tmpfs/src/github/python-docs-samples/bigquery/cloud-client/.nox/py-3-7/lib/python3.7/site-packages/google/api_core/retry.py", line 184, in retry_target
    return target()
  File "/tmpfs/src/github/python-docs-samples/bigquery/cloud-client/.nox/py-3-7/lib/python3.7/site-packages/google/cloud/_http.py", line 423, in api_request
    raise exceptions.from_http_response(response)
google.api_core.exceptions.PreconditionFailed: 412 POST https://bigquery.googleapis.com/bigquery/v2/projects/python-docs-samples-tests/datasets: IAM setPolicy failed for Dataset python-docs-samples-tests:natality_regression_6f9938f5_60fe_47c0_9424_41d8d1b3db7a: There were concurrent policy changes. Please retry the whole read-modify-write with exponential backoff.

[tmatsuo]

@busunkim96 I filed it as a flake at #3882

[tmatsuo]

There's a failure, but it's a kind of flakes. The flake is reported. I'm going to merge this with admin power.