This DR revisits my older code in D729. It uses Pungi4
and doesn't download disk image by itself, it let's GRU to do
it. It now also downloads vmlinuz and initrd so that it wouldn't
have to extract them from disk image.
Pull request from https://github.com/os-autoinst/openQA/pull/627
is needed, because _URL isn't enough by itself - disk images are
compressed, so I've added possibility to append _URL_EXTRACT to
parameter and it then extracts downloaded image.
For The Future, perhaps we'd prefer to have _get_dkboot_urls(location, arch='armhfp') or something?