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/volume/util/util_test.go

@@ -164,7 +164,7 @@ metadata:
   name: testpod
 spec:
   containers:
-    - image: gcr.io/google_containers/busybox
+    - image: k8s.gcr.io/busybox
 `,
 			false,
 		},
@@ -181,7 +181,7 @@ spec:
   "spec": {
     "containers": [
       {
-        "image": "gcr.io/google_containers/busybox"
+        "image": "k8s.gcr.io/busybox"
       }
     ]
   }
@@ -197,7 +197,7 @@ kind: Pod
 metadata:
   name: testpod
 spec:
-  - image: gcr.io/google_containers/busybox
+  - image: k8s.gcr.io/busybox
 `,
 			true,
 		},