rax module: Support config_drive and user_data

This pull request adds attributes for `config_drive` and `user_data`.  This is primarily useful for utilizing cloudinit.

The `user_data` attribute can be a string or a file path.  If `user_data` is supplied on it's own, it implies `config_drive`.  `config_drive` can be used on it's own.

Here is an example task:

```yaml
    - name: Build CoreOS Servers
      rax:
        credentials: ~/.raxpub
        name: coreos%02d
        flavor: performance1-1
        image: 70210a69-081e-4a5e-b402-3cdbabfdcddd
        region: ORD
        state: present
        count: 3
        exact_count: yes
        group: coreos
        wait: yes
        user_data: files/coreos-cloud-config.yml
```