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/integration/framework/util.go

@@ -31,7 +31,7 @@ import (
 const (
 	// When these values are updated, also update cmd/kubelet/app/options/container_runtime.go
 	// A copy of these values exist in test/utils/image/manifest.go
-	currentPodInfraContainerImageName    = "gcr.io/google_containers/pause"
+	currentPodInfraContainerImageName    = "k8s.gcr.io/pause"
 	currentPodInfraContainerImageVersion = "3.1"
 )