configdrive: Use the EC2 metadata over OpenStack

WIP

Still needs to be properly tested.
We need to check `openstack/latest/user_data` as well. I'm fine with preferring the versioned copy and requiring `meta_data.json` to be versoned sounds sane but for the sake of simplicity `latest` is always what we've used for `user_data` in hand-constructed config drives:

https://github.com/coreos/coreos-cloudinit/blob/master/Documentation/config-drive.md