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/metadata-proxy/gce/metadata-proxy.yaml

@@ -38,7 +38,7 @@ spec:
       dnsPolicy: Default
       containers:
       - name: metadata-proxy
-        image: gcr.io/google_containers/metadata-proxy:v0.1.9
+        image: k8s.gcr.io/metadata-proxy:v0.1.9
         securityContext:
           privileged: true
         # Request and limit resources to get guaranteed QoS.
@@ -51,7 +51,7 @@ spec:
             cpu: "30m"
       # BEGIN_PROMETHEUS_TO_SD
       - name: prometheus-to-sd-exporter
-        image: gcr.io/google_containers/prometheus-to-sd:v0.2.2
+        image: k8s.gcr.io/prometheus-to-sd:v0.2.2
         # Request and limit resources to get guaranteed QoS.
         resources:
           requests: