(FACT-643) Cherry-pick lost Rackspace fact back into master

This commit cherry-picks work done on the rackspace fact which
was lost during the facter-2 / master merge.

Original commit (a6bc494d4):
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