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

@ryanmoran -- please review and confirm.  Thanks.