Switch to k8s.gcr.io vanity domain

This is the 2nd attempt.  The previous was reverted while we figured out
the regional mirrors (oops).

New plan: k8s.gcr.io is a read-only facade that auto-detects your source
region (us, eu, or asia for now) and pulls from the closest.  To publish
an image, push k8s-staging.gcr.io and it will be synced to the regionals
automatically (similar to today).  For now the staging is an alias to
gcr.io/google_containers (the legacy URL).

When we move off of google-owned projects (working on it), then we just
do a one-time sync, and change the google-internal config, and nobody
outside should notice.

We can, in parallel, change the auto-sync into a manual sync - send a PR
to "promote" something from staging, and a bot activates it.  Nice and
visible, easy to keep track of.

pkg/api/testing/deep_copy_test.go

@@ -64,7 +64,7 @@ var benchmarkPod api.Pod = api.Pod{
 		Containers: []api.Container{
 			{
 				Name:  "etcd-container",
-				Image: "gcr.io/google_containers/etcd:2.0.9",
+				Image: "k8s.gcr.io/etcd:2.0.9",
 				Command: []string{
 					"/usr/local/bin/etcd",
 					"--addr",
@@ -120,7 +120,7 @@ var benchmarkPod api.Pod = api.Pod{
 				},
 				Ready:        true,
 				RestartCount: 0,
-				Image:        "gcr.io/google_containers/etcd:2.0.9",
+				Image:        "k8s.gcr.io/etcd:2.0.9",
 				ImageID:      "docker://b6b9a86dc06aa1361357ca1b105feba961f6a4145adca6c54e142c0be0fe87b0",
 				ContainerID:  "docker://3cbbf818f1addfc252957b4504f56ef2907a313fe6afc47fc75373674255d46d",
 			},

pkg/api/testing/replication_controller_example.json

@@ -47,7 +47,7 @@
                 "containers": [
                     {
                         "name": "elasticsearch-logging",
-                        "image": "gcr.io/google_containers/elasticsearch:1.0",
+                        "image": "k8s.gcr.io/elasticsearch:1.0",
                         "ports": [
                             {
                                 "name": "db",