Re-enable offline installation of a combi server including diskless

workstation support. (Closes: #867271, #904331)

015-edu-apt-source: fix apt-get options to be able to use a repo of
type 'file://'. As 'media/cdrom/' in the LTSP chroot is treated as
such a repo, add 'acquire::check-valid-until=0' to APT_GET_OPTS;
otherwise installation fails because the Release file is expired.

032-edu-pkgs: Move all diskless workstation installation parts to
the finalization stage of LTSP chroot installation.