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.
counterpoint: It's much harder to add fleet metadata after the fact, so having this here costs nothing but provides a benefit later if you want to run fleet commands on the cluster segregated by kubernetes_role.