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.
And now I actually notice that this is AtomicUpdate... carry on :)