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.
I'd take a closer look at some point about making the failure modes for these scripts a little more robust.  As things stand if, say, etcdctl fails it'll be hard to figure out what is going on.