lib/ Use the new usable auth feature

I spoke with the Google API Python client library authors. They confirmed that a
complete solution for service account json keys is not yet available, but there
is already a publich API that we can use in non-interpersonate cases.
GoogleCredentials implements whatever I had to manually do for json key parsing.

TEST=Unit tests in CL:299694 and a trybot run against a config that has GCE
tests enabled.

Change-Id: I7180463b1fb392e90068e0999cd47462ce1b1419
Commit-Ready: Daniel Wang <>
Tested-by: Daniel Wang <>
Reviewed-by: Fang Deng <>
1 file changed