Cleanly delete service without endpoints key

If a service is deleted before the etcd key corresponding to its
endpoints has been created, a 500 error is returned (etcd key
not found).  Instead, key not found should be ignored silently.

There is a potential race condition with the controller creating
endpoints after the service has been deleted which a cleanup task
should handle in the future.

Closes #684
Merge pull request #685 from smarterclayton/error_on_empty_service

Cleanly delete service without endpoints key