gce_au_worker: Attempt to delete resources only when they exist

There have been failures found in recent trybot runs that were due to a second
attempt to delete a GCE resource. GCEAUWorker creates three types of resources -
GCS object, GCE images and GCE instances. Need to check their existence before
making the remote calls.

TEST=gce_au_worker_unittest.py and trybot against lakitu-pre-cq

Change-Id: I8c52e7fd5a751fe80d6c77c43d509e1e31567443
Reviewed-on: https://chromium-review.googlesource.com/310077
Commit-Ready: Daniel Wang <wonderfly@google.com>
Tested-by: Daniel Wang <wonderfly@google.com>
Reviewed-by: Simran Basi <sbasi@chromium.org>
3 files changed