Add update to the pod etcd handler.

It's not racy - it'll reject the write if the version changes in between (SetObj compareandswaps), whereas atomic just tries to keep merging.  So technically this path is more correct right now than AtomicUpdate which should really be AtomicMergeHelper

----- Original Message -----
> > @@ -198,7 +198,37 @@ func (r *Registry) assignPod(podID string, machine
> > string) error {
> >  }
> >  
> >  func (r *Registry) UpdatePod(ctx api.Context, pod *api.Pod) error {
> > -	return fmt.Errorf("unimplemented!")
> > +	var podOut api.Pod
> > +	podKey := makePodKey(pod.ID)
> > +	err := r.EtcdHelper.ExtractObj(podKey, &podOut, false)
> 
> From here to the set should be in an AtomicUpdate function. As written it's
> racy. The atomic update thingy will do the read for you.
> 
> ---
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> https://github.com/GoogleCloudPlatform/kubernetes/pull/1713/files#r18782464