Added functionality for users to specify networks

- This commit allows a user to specify optional additional networks,
  allowing them to join their test servers to a user-created network
  inside of Rackspace.

- By default, all servers are joined to PublicNet
  ('00000000-0000-0000-0000-000000000000') and ServiceNet
  ('11111111-1111-1111-1111-111111111111').

- To save users from having to specify these two mandatory networks, the
  method #rackpsace_networks will simply append these two networks to
  whatever networks the user has specified in the kitchen.yml file.

- Appropriate, but slightly honey-badger (what law of demeter?) specs
  have been added for this new functionality.

- This has been tested by me and a couple of other users in my company.
  It probably works.

Updated README to include networks: stanza.

Clean up slightly abusive trace message.

Make RuboCop happ(ier).

- I really don't know how to trim this down to 100 lines. Any ideas?
Do a little cleanup for RuboCop
Merge pull request #33 from test-kitchen/roboticcheese/custom_networks

Support custom networks