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.
Oh, is this necessary?  I thought I was seeing that systemd was clever enough to realise the EnvironmentFile had changed, and restarted the master-apiserver.service unit.