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.
In several of the other services, we just have `Extract`. I know that there is one that doesn't fit that mold (`ExtractDiskConfig`) but for consistency, we should either change those to reflect this way, or vice versa.