Minimal status mutation change

PUT /api/v1beta3/namespaces/default/pods/foo/status
    {
      "metadata": {...}, // allowed for valid values
      "spec": {}, // ignored
      "status": {...}, // allowed, except for Host
    }

Exposes the simplest possibly change. Needs a slight refactoring
to RESTUpdateStrategy to split merging which can be done in a
follow up.

pkg/registry/etcd/etcd_test.go

@@ -41,7 +41,7 @@ func NewTestEtcdRegistry(client tools.EtcdClient) *Registry {
 
 func NewTestEtcdRegistryWithPods(client tools.EtcdClient) *Registry {
 	helper := tools.EtcdHelper{client, latest.Codec, tools.RuntimeVersionAdapter{latest.ResourceVersioner}}
-	podStorage, _ := podetcd.NewREST(helper, nil)
+	podStorage, _, _ := podetcd.NewREST(helper, nil)
 	registry := NewRegistry(helper, pod.NewRegistry(podStorage))
 	return registry
 }