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.
AtomicUpdate isn't meant to be called directly by PUT. Take a look at where it's used:

https://github.com/GoogleCloudPlatform/kubernetes/blob/b0d18b2af05ea81af84f876f00ec9916190581bb/pkg/registry/etcd_registry.go#L115

AtomicUpdate is the general pattern of read, apply delta, attempt to update. Application code is invoked to apply the delta. If application overwrites the object instead, that's an application code bug.