Add initial functionality for exposing Rackspace Cloud data

Rackspace Cloud doesn't expose the metadata service provided
by OpenStack.  You can get some basic information about region
and what the instance id is from xenstore, though
Set is_rsc to nil (or don't set it) if we're not on Rackspace Cloud
Merge pull request #436 from katzj/rackspace-cloud-facts

Add initial functionality for exposing Rackspace Cloud data (#20468)