Use atomic create in EtcdHelper.AtomicUpdate

Before this CL, AtomicUpdate was only "atomic" on update operations where the entry already exist for given key.
If multiple AtomicUpdate were issued concurrently, all could succeed.

This CL fixes the issue by using EtcdClient.Create when creating new entry, which ensures that no previous entry exist.
But that's application logic, not etcd.  Isn't a create an indication of intent?  If I want createOrUpdate semantics that is what PUT is for, and is decided at a restful level (rest storage), not in the etcd layer.