Support boot from volume in kitchen-rackspace

[martinb3]

Similar to this issue in knife-rackspace: chef-boneyard/knife-rackspace#91. We should also probably support boot from volume cloud server flavors. Putting in this issue so we don't forget :)

I'd like to work on this if possible, I'm a ruby dev at rackspace currently that uses chef/test-kitchen quite a bit.