Add a GCE au_worker

Add a gce_au_worker that runs VMTests on GCE instances.
Extend cros_au_test_harness.py and ctest.py to accept a DUT type of 'gce'.
Add unit tests.

gce_au_worker mimics vm_au_worker, but creates a GCE instance at every call of
UpdateImage. Old images/instances will be deleted at the end of the test. And
like real_au_worker, it calls directly to 'test_that' for the actual autotest
interaction, without any interim external scripts.

This CL uses Lakitu's GCE private project and it should be replaced by one
that's managed by ChromeOS Infrastructure team.

This CL includes an end-to-end test that runs SimpleTestVerify and
SimpleTestUpdateAndVerify in parallel with gce_au_worker. It servces as a demo
but includes some private information that should be stripped before submission.
The tests for update with payloads are coming in a follow up CL.

BUG=brillo:1196
BUG=brillo:1197
TEST=cros_au_test_harness_unittest.py

Change-Id: I5c1104ec28a124c3d85bc7b62a1686a401fb2e95
Reviewed-on: https://chromium-review.googlesource.com/278012
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Daniel Wang <wonderfly@google.com>
Tested-by: Daniel Wang <wonderfly@google.com>
7 files changed