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 avoid hard coding this as it won't work as we do new releases, etc.  Instead I'd stage the release stuff you need into swift.

The only tar you need is probably is `kubernetes-server-linux-amd64.tar.gz` with all of the server bins.  I'd do the following:

* Search for this in two places.  When building from git this'll end up in `${KUBE_ROOT}/_output/release-tars/kubernetes-server-linux-amd64.tar.gz`.  With a binary release it'll end up in `${KUBE_ROOT}/server/kubernetes-server-linux-amd64.tar.gz`.  You can see where we do this for GCE here: https://github.com/GoogleCloudPlatform/kubernetes/blob/master/cluster/gce/util.sh#L52
* Upload this tar file either directly to your servers or stage it through your object store.  We stage it through GCS here: 
https://github.com/GoogleCloudPlatform/kubernetes/blob/master/cluster/gce/util.sh#L101

If we do this right, then users can develop k8s against rackspace easily and the release they `kube-up` will be the one they download -- with matched client tools and all.