Validates that we can only authenticate to one S3 and one GCS.

def validate_stores_secrets_keys(stores_secrets):
        """Validates that we can only authenticate to one S3 and one GCS."""
        stores = set([])
        for store_secret in stores_secrets:
            if store_secret['store'] in stores:
                raise TensorboardValidation('Received an invalid store configuration.')
            elif store_secret['store'] not in {GCS, S3}:
                raise TensorboardValidation('Received an unsupported store configuration.')
            stores.add(store_secret['store'])