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.
I think a sync.WaitGroup might make more sense for this syncronization. Also, I think you should be able to do this with a single go routine function that you start multiple times in a loop. Note that there'd still be a collision even if both groups only incremented by one.