[LIBCLOUD-356] Add config_drive to support user_data

I'm not actually sure if we want to always send `config_drive` attribute when `user_data` is provided.

I believe older versions and some installations of OpenStack support `user_data` without `config_driver`.

If that is in fact the case, we should also add `ex_config_drive` argument to the method (and this argument only makes sense if `ex_userdata` is provided as well).

/cc @briancurtin @alex