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.

hack/lib/swagger.sh

@@ -117,7 +117,7 @@ kube::swagger::gen_api_ref_docs() {
       -v "${swagger_spec_path}":/swagger-source:z \
       -v "${register_file}":/register.go:z \
       --net=host -e "https_proxy=${KUBERNETES_HTTPS_PROXY:-}" \
-      gcr.io/google_containers/gen-swagger-docs:v8 \
+      k8s.gcr.io/gen-swagger-docs:v8 \
       "${swagger_json_name}"
   done