Add details about allowed provisioner values

Currently gophercloud only accepts `rackspace-us` or `rackspace-uk` as provider values[1] although there is a pull request to allow arbitrary endpoints[2]

[1](https://github.com/rackspace/gophercloud/blob/master/global_context.go#L14)
[2](https://github.com/rackspace/gophercloud/pull/84)