gce_au_worker: Shard tests across zones to prevent from zonal outages

If for some reason an instance cannot be created in a particular zone,
try other zones (currently all in the same region) until we succeed.

TEST=trybot against lakitu-pre-cq

Change-Id: I6d1a3b300ae8c9c4c5ddd8171c494ecc587010b6
Reviewed-on: https://chromium-review.googlesource.com/630018
Commit-Ready: Daniel Wang <wonderfly@google.com>
Tested-by: Daniel Wang <wonderfly@google.com>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
1 file changed