Adds rackspace cloudnetworks-us provider

The endpoint has to be automatically provided to the user. The problem is some small inconsistencies in how providers and services manage versioning. I think (my guess) in the end something similar to https://github.com/jclouds/jclouds-labs-openstack/blob/master/openstack-glance/src/main/java/org/jclouds/openstack/glance/functions/RegionToEndpointNegotiateVersion.java will have to be moved up to better negotiate versioning for services.