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.
@doublerr You shouldn't need to touch anything in `build/`.  We really have two separate places now where we upload stuff -- it keeps things cleaner and keeps the build/release stuff completely divorced from the "run a cluster" stuff.

* `build/release.sh` -- Here we are uploading the binary release tarball for doing things like automated releases and nightlies.  Unless you are going to be cutting and publishing your own releases (or unless we want to mirror releases into rackspace) there is no reason to touch this.  Most developers won't upload here as part of building dev releases
* `cluster/*/util.sh` -- Here we *assume* that there is a tarball with the built stuff we need locally.  We need to get it to the machines we are bringing up and deploying.  You may want to upload through some cloud storage as a easy way to get those tar files there -- that is what we do for GCE.  But for something like vagrant, we take advantage of the shared `/vagrant` file system.  For vSphere, we `scp` the tars up over an ssh connection.

While this is a little less efficient than it could be (in the binary release case the tars will transit your local workstation and have to be downloaded and then re-uploaded) it really smooths over the dev situation and makes it super easy to distribute a single tarball that has everything you need.

We may short-circuit the re-upload in the future but I consider that an optimization over the pattern we have now.

Sorry this is so confusing!