Media provider unique ID does not work for Debian-based distros
Debian installation media have no variables in it (e.g. http://archive.ubuntu.com/ubuntu) so the unique id is calculated the same for all operating systems and does not actually work. We need to generate it from pxedir, e.g. http://sg.archive.ubuntu.com/dists/main/installer-x86_64/current/images/netboot/ubuntu-installer/x86_64.