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.

build/build-image/cross/Makefile

@@ -21,7 +21,7 @@ TAG=$(shell cat VERSION)
 all: push
 
 build:
-	docker build --pull -t gcr.io/google_containers/$(IMAGE):$(TAG) .
+	docker build --pull -t staging-k8s.gcr.io/$(IMAGE):$(TAG) .
 
 push: build
-	gcloud docker -- push gcr.io/google_containers/$(IMAGE):$(TAG)
+	gcloud docker -- push staging-k8s.gcr.io/$(IMAGE):$(TAG)