Update documentation and change rackspace_vars.yaml to an untracked file...

Since everyone will have a different rackspace_vars.yaml, I renamed it to rackspace_vars.yaml.example and updated the docs.

I also updated the docs with a general description and information on the rax module changes necessary for this to run.

If you prefer just the description / notes update while keeping rackspace_vars.yaml tracked, I'd be happy to resubmit.