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

addons/nginx-ingress/google-cloud/default-backend/deployment.yaml

@@ -20,7 +20,7 @@ spec:
           # Any image is permissable as long as:
           # 1. It serves a 404 page at /
           # 2. It serves 200 on a /healthz endpoint
-          image: gcr.io/google_containers/defaultbackend:1.4
+          image: k8s.gcr.io/defaultbackend:1.4
           ports:
             - containerPort: 8080
           resources: