release: add basic cloudinit images

Provide both zfs and ufs images which a 1MB partition reserved for the
config drive wearing a GPT Label "config-drive" to allow consumer to
know where they should push the config drive on the provided image.

2 formats available: qcow2 and raw

This has been tested on OVHCloud baremetal via "bring your own image"
Also tested on openstack

Reviewed by: emaste
Sponsored by: OVHCloud
Differential Revision: https://reviews.freebsd.org/D44369