Add authentication docs for Bigtable

We should expand [Bigtable documentation](http://docs.janusgraph.org/latest/bigtable.html) to cover authentication:

* when running on Google Cloud, whether Compute Engine or Container Engine, the default VM scopes provide service account credentials automatically, so it just works out of the box

* when running outside of Google Cloud, one must explicitly provide a service account by setting the environment variable `GOOGLE_APPLICATION_CREDENTIALS` to point to the account file; this is described on https://cloud.google.com/docs/authentication/getting-started and needs to be documented for clarity