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.
To describe this a bit more, here's where I had been: https://github.com/smarterclayton/kubernetes/commit/d531c20823857c004447df4879c94c78f0a05702#diff-e5070ab9f9591f66b1aebf7bd6766637R168 and https://github.com/smarterclayton/kubernetes/commit/d531c20823857c004447df4879c94c78f0a05702#diff-d625616d264f6f8353cc2045d4c344aaR229

The latter indicates the POST request being rejected because two inflight requests from different users being conflicting.  If Create() in etcd_tools eats the conflict, then you are in LWW.