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/workers/cl/worker.yaml.tmpl

@@ -90,7 +90,7 @@ 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
@@ -109,7 +109,7 @@ storage:
             --volume config,kind=host,source=/etc/kubernetes \
             --mount volume=config,target=/etc/kubernetes \
             --insecure-options=image \
-            docker://gcr.io/google_containers/hyperkube:v1.10.0 \
+            docker://k8s.gcr.io/hyperkube:v1.10.0 \
             --net=host \
             --dns=host \
             --exec=/kubectl -- --kubeconfig=/etc/kubernetes/kubeconfig delete node $(hostname)