cloudware: allow disk format to be a list

Make basic-cloudinit available both in qcow2 and raw formats

MFC After: 3 days
Co-Authored-By: bapt
Sponsored by: OVHCloud (original patch from bapt)
Differential Revision: https://reviews.freebsd.org/D44747

(cherry picked from commit 5c213bf01a5dca7c4030efd2ad9c9f84852e7b32)