This allows the etcd docker registry that is currently hard coded to
`gcr.io/google_containers/etcd` in the `etcd.manifest` template to be
overridden.  This can be used to test new versions of etcd with
kubernetes that have not yet been published to
`gcr.io/google_containers/etcd` and also enables cluster operators to
manage the etcd images used by their cluster in an internal
repository.