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.
BTW, I don't think this is actually going to work unless you make key not found an acceptable error in the read step above.