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.

test/e2e_node/node_problem_detector_linux.go

@@ -45,7 +45,7 @@ var _ = framework.KubeDescribe("NodeProblemDetector", func() {
 		pollInterval   = 1 * time.Second
 		pollConsistent = 5 * time.Second
 		pollTimeout    = 1 * time.Minute
-		image          = "gcr.io/google_containers/node-problem-detector:v0.4.1"
+		image          = "k8s.gcr.io/node-problem-detector:v0.4.1"
 	)
 	f := framework.NewDefaultFramework("node-problem-detector")
 	var c clientset.Interface