gce_au_worker: Revise gce_au_worker based on the new GCE util

All changes include:

* Update gce_au_worker.py with the lately added GCE Python API based utility,
  i.e., gce.py, see CL:300202.
* Use locally built GCE tarball instead of depending on its existence in the
  archive GCS bucket, see CL:300315.
* Update lib/constants with the credentials of the GCE project used for testing.
  Background: brbug.com/1199 and crbug.com/526292.
* Handle test failures by archiving GCE tarball and test private key for repro.
* Add a unit test for gce_au_worker.py.

TEST=Trybot run with a config that enables gce tests.

Change-Id: Ide8ed380f23e1c1f015094413f5bc93cbc60760f
Reviewed-on: https://chromium-review.googlesource.com/301741
Commit-Ready: Daniel Wang <wonderfly@google.com>
Tested-by: Daniel Wang <wonderfly@google.com>
Reviewed-by: Simran Basi <sbasi@chromium.org>
5 files changed