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.

cluster/addons/metrics-server/metrics-server-deployment.yaml

@@ -47,7 +47,7 @@ spec:
       serviceAccountName: metrics-server
       containers:
       - name: metrics-server
-        image: gcr.io/google_containers/metrics-server-amd64:v0.2.1
+        image: k8s.gcr.io/metrics-server-amd64:v0.2.1
         command:
         - /metrics-server
         - --source=kubernetes.summary_api:''
@@ -56,7 +56,7 @@ spec:
           name: https
           protocol: TCP
       - name: metrics-server-nanny
-        image: gcr.io/google_containers/addon-resizer:1.8.1
+        image: k8s.gcr.io/addon-resizer:1.8.1
         resources:
           limits:
             cpu: 100m