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).
Can I get you to change the fog dependency version to `1.22` like so

```ruby
gem.add_runtime_dependency "fog", "~> 1.22"
```

That way ruby gems will pick up patch releases. 

BTW, you did a great job on this PR. Once you make this one minor change, I will will merge it in.