Extension: Disk Config

Implementing the [disk-config extension](http://developer.openstack.org/api-ref-compute-v2-ext.html#ext-os-disk-config-servers) for OpenStack and Rackspace compute.
Oh, ok. I'm fine with having it like that. I'm just thinking that having 2 ways of writing an extraction will force the users to know if they're using an extension or not.