rackspace command and subcommands (list images and flavors)

I wanted to get an idea of how to add commands within the Rackspace provider that re-use existing middleware, like ConnectRackspace to get the Fog::Compute connection.  I haven't added tests or beautified the output, but I wanted to share a simple working solution to get feedback.

This adds the commands:
- vagrant rackspace images
- vagrant rackspace flavors

(Remember that in this repo you'll need to use "bundle exec", but normal vagrant-rackspace users won't)

Also, multi-machine settings do work.  In a multiple machine environment (like the Vagrantfile.multi sample I added) the command without sub arguments will display the images (or flavors) available for each machine.  In the Vagrantfile.multi example the machines are named after regions, so you can view images or flavors for just the machine named dfw with:
- vagrant rackspace images dfw
- vagrant rackspace flavors dfw