Support for GCS buckets as default storage locations

[henxing]

For upload, storage, and backup, default buckets may be specified in the helm/tator/values.yaml file, which creates environment variables read by get_tator_store to communicate with the storage server. Currently, GCS buckets may only be used as organization-specific buckets. Add the necessary values to values.yaml to create an environment variable containing the GCS key info (a json string) and update get_tator_store to look for them, too.