swift backup driver not using service tokens

when using per_user auth type (as opposed to single_user), the user's password is unknown, so swiftclient is not able to retry auth if the user's token expires causing an HTTP 401. It is important that cinder provide service credentials to swiftclient in this case, so that the service token (which would have known credentials and therefore can be reauthed) is used for checking expiration.

found in master