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.

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

(cherry picked from commit 78b6117dac99c80fe618979fef4ef322b97415fa)
(cherry picked from commit a977444652a59332c53b652b912c38a1d1802b58)