Rackspace - Switch to CoreOS for standard cluster

The Rackspace setup for Kubernetes now leverages CoreOS instead of Ubuntu. We've dropped Salt completely for the cluster.

This doesn't include the updated release scripts so I've left those in the icebox for now.
@jbeda I'm not sure what to do here. You switch between `gce` and `gcs` acronyms in cluster and build respectively. This provider logic won't work.