'Get a credential from the multistore.
The multistore must be locked.
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:
The credential specified or None if not present'

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

    key = (client_id, user_agent, scope)
    return self._data.get(key, None)
