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.

api/openapi-spec/swagger.json

@@ -76545,7 +76545,7 @@
     ],
     "properties": {
      "names": {
-      "description": "Names by which this image is known. e.g. [\"gcr.io/google_containers/hyperkube:v1.0.7\", \"dockerhub.io/google_containers/hyperkube:v1.0.7\"]",
+      "description": "Names by which this image is known. e.g. [\"k8s.gcr.io/hyperkube:v1.0.7\", \"dockerhub.io/google_containers/hyperkube:v1.0.7\"]",
       "type": "array",
       "items": {
        "type": "string"

api/swagger-spec/v1.json

@@ -20261,7 +20261,7 @@
       "items": {
        "type": "string"
       },
-      "description": "Names by which this image is known. e.g. [\"gcr.io/google_containers/hyperkube:v1.0.7\", \"dockerhub.io/google_containers/hyperkube:v1.0.7\"]"
+      "description": "Names by which this image is known. e.g. [\"k8s.gcr.io/hyperkube:v1.0.7\", \"dockerhub.io/google_containers/hyperkube:v1.0.7\"]"
      },
      "sizeBytes": {
       "type": "integer",