Added the ability to assign networks by name in the Rackspace module

This is a bit different than the rest of the Rackspace module where everything had to go by ID (like flavors and images). I personally find that a bit pedantic because I always have to go hunting for the IDs and in some cases view page sources (although I suppose I could just leverage the API). Instead, I simply allow networks to be referenced by name with special, reserved names for 'public' and 'private'.

It's pretty trivial to change to reference these by id instead so just let me know if I should update this. I also wasn't sure how to do tests for this, but I tested locally and it works ok on my end.