Rax mod utils improvements

This pull allows the following behaviors:

1) Support other OpenStack clouds with the rax module
2) Support pyrax environments via `~/.pyrax.cfg`
3) Support keyring based authentication when keyring_username is found in the environment from `~/.pyrax.cfg` or when the api_key is set to 'USE_KEYRING'
4) Update rax DOCUMENTATION with new auth related attributes