'Get a Storage object to get/set a credential.
This Storage is a \'view\' into the multistore.
Args:
client_id: The client_id for the credential
user_agent: The user agent for the credential
scope: A string for the scope(s) being requested
Returns:
A Storage object that can be used to get/set this cred'

def _get_storage(self, client_id, user_agent, scope):

    return self._Storage(self, client_id, user_agent, scope)
