Automatic merge from submit-queue

Remove the Rackspace provider

**What this PR does / why we need it**:

To aid the effort of moving providers out of the cluster dir, I'm
removing Rackspace and leaving behind a README.md simply as a
placeholder until the entire dir is deleted.

**Which issue this PR fixes**

Fixes #6962

**Release note**:

```release-note
Deployment of Kubernetes clusters on Rackspace using the in-tree bash deployment (i.e. cluster/kube-up.sh or get-kube.sh) is obsolete and support has been removed.```