Module patch

Return the endpoint from config or the catalog.

THIS IS FROM os_client_config.CloudConfig.get_session_endpoint()
If a configuration lists an explicit endpoint for a service,
return that. Otherwise, fetch the service catalog from the
keystone session and return the appropriate endpoint.

:param service_key: Generic key for service, such as 'compute' or
                    'network'

:returns: Endpoint for the service, or None if not found