Fix #2540: Error listing VM's on Rackspace CR's

This PR fixes the issue with listing VM's, but it also:
* Moves from the deprecated `:rackspace_endpoint` to `:rackspace_compute_url`
* Suggests for users to choose the v2 API (which is what we require)

I'm happy to move those two things into separate PR's, but since they are related to this fix I figured they should at least get reviewed along with it.