support "user_data" and "config_drive" configuration options

Fog supports the `config_drive` option as of `1.16.0` and the `user_data` option as of `1.22.0`. Both options need to be set, for example, if one desires to deploy a CoreOS image on the Rackspace Cloud.

My "test" consisted of applying these same changes to the vagrant-rackspace plugin in `~/.vagrant.d/gems/gems/` (along with the changes in #99), and then successfully bringing online a CoreOS image with `user_data` adapted from the [CoreOS website docs for Rackspace](https://coreos.com/docs/running-coreos/cloud-providers/rackspace/).

I also ran `bundle exec rake` and all the tests passed, which is a good sign I suppose (the fog version bump didn't break anything), though I did not write any specs for my changes *per se* (I don't really know ruby).