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.
This is an extension, though. I was following [@jamiehannaford's lead](https://github.com/rackspace/gophercloud/blob/v0.2.0/openstack/networking/v2/extensions/external/results.go#L54-L66) on naming here.