Fix saml_login to work with cf-registrar.

* The config key of 'mbus' was changed to 'message_bus_servers':
    https://github.com/cloudfoundry/vcap-common/blob/86c1201162dc8b2938d7abac3c1f45978e852502/bin/cf-registrar#L32
* This change also makes the saml login config consistent with the login server's config.