Revert "Revert "Use atomic create in EtcdHelper.AtomicUpdate""

This reverts commit 73b64aa490e6ae4cc9f5c0026e0d6f8bf03bbdf5.
Remove duplicated def of IsEtcdNodeExist
Merge pull request #794 from nyaxt/reland_atomicetcd

Remove duplicated defs and reland #694.