Support data endpoint and admin endpoint overrides from gcloud

[rameshdharan]

Currently we support reading the configured Cloud Bigtable project from gcloud.

We should also support reading the api_endpoint_overrides/bigtable and api_endpoint_overrides/bigtableadmin values, if specified in gcloud configuration.

See https://cloud.google.com/bigtable/docs/regional-endpoints#how-to for a case where this would be useful.