Allow external login server

Allow the following properties to be set via stub:
- [login.url](https://github.com/cloudfoundry/cf-release/blob/master/jobs/cloud_controller_ng/spec#L317-L318)
- [uaa.login.client_secret](https://github.com/cloudfoundry/cf-release/blob/master/jobs/uaa/spec#L117-L118)