17f2449821
Implementing the [google default creds](https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md#google_default_credentials) interop test for go. This test case was added in https://github.com/grpc/grpc/pull/17213. The existing OAuth creds test cases turned out to not be great first for google default creds testing, so https://github.com/grpc/grpc/pull/17213 got rid of those tests and created a new dedicated test for google default creds. This PR does that and also gets rid of the now-unneeded (we thought they would be needed earlier) OAuth test scenarios with plain ALTS.