gRPC pubsub missing scope error

[daspecster]

It appears that something changed in gRPC.

Exception google.auth.exceptions.RefreshError: RefreshError('invalid_scope: Empty or missing scope not allowed.', u'{\n  "error" : "invalid_scope",\n  "error_description" : "Empty or missing scope not allowed."\n}') in 'grpc._cython.cygrpc.plugin_get_metadata' ignored

Reference: https://travis-ci.org/GoogleCloudPlatform/google-cloud-python/builds/187483537#L2101

Running $ tox -e system-tests -- pubsub yields the same results.

However running $ GOOGLE_CLOUD_DISABLE_GRPC=true tox -e system-tests -- pubsub allows the tests to pass.

[dhermes]

That error is not in gRPC, it's in the auth code. For some reason the credentials aren't scoped. Might be related to #2870 or #2874 (I am author of both)