Use k8s.gcr.io instead of gcr.io/google_containers

* Kubernetes recommends using the alias to fetch images
from the nearest GCR regional mirror, to abstract the use
of GCR, and to drop names containing 'google'
* https://groups.google.com/forum/#!msg/kubernetes-dev/ytjk_rNrTa0/3EFUHvovCAAJ

google-cloud/container-linux/kubernetes/controllers/cl/controller.yaml.tmpl

@@ -118,7 +118,7 @@ storage:
       mode: 0644
       contents:
         inline: |
-          KUBELET_IMAGE_URL=docker://gcr.io/google_containers/hyperkube
+          KUBELET_IMAGE_URL=docker://k8s.gcr.io/hyperkube
           KUBELET_IMAGE_TAG=v1.10.0
     - path: /etc/sysctl.d/max-user-watches.conf
       filesystem: root