backfill images from google-containers

This is part of
https://github.com/kubernetes/k8s.io/blob/master/k8s.gcr.io/Vanity-Domain-Flip.md,
in preparation for the domain flip. Backfilling all images to the
top-level (root) directory ensures that the transition for existing
images post-flip will be painless.

Author: Linus Arver