add rackspace support

[josephholsten]

Adds minimal rackspace support. Just add this to the top of your .kitchen.yml:

---
driver_plugin: vagrant
driver_config:
  use_vagrant_berkshelf_plugin: true
  provider: rackspace
  customize:
    username: <%= ENV['RS_USERNAME'] %>
    api_key: <%= ENV['RS_API_KEY'] %>

[josephholsten]

@fnichol is there anything I should do to help you merge this? Tests, docs, beer?

[fnichol]

@josephholsten derp, it'll help if I track GitHub notifications :( I owe you beers, thank you!