Added support for creating and retrieving user-provided service instances.

@ramnivas The "pragmatic" approach ended up being easier than we discussed. Credentials for user-provided service instances should not be part of the object model, since credentials for system-provisioned services are not returned in the object model. So user-provided credentials are provided only to the `createUserProvidedService()` method. Just some changes to the REST response mapping code and a small helper in the model.