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/device-plugins/nvidia-gpu/daemonset.yaml

@@ -36,7 +36,7 @@ spec:
         hostPath:
           path: /dev
       containers:
-      - image: "gcr.io/google-containers/nvidia-gpu-device-plugin@sha256:0e79da6998a61257585e0d3fb5848240129f0fa5b4ad972dfed4049448093c33"
+      - image: "k8s.gcr.io/nvidia-gpu-device-plugin@sha256:0e79da6998a61257585e0d3fb5848240129f0fa5b4ad972dfed4049448093c33"
         command: ["/usr/bin/nvidia-gpu-device-plugin", "-logtostderr"]
         name: nvidia-gpu-device-plugin
         resources: