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.
AtomicUpdate is called by PUT. POST uses SetObj, which should start using create, I think.

Note that AtomicUpdate executes application logic in a loop.