guest-os: Add JeOS 20 config file and kickstart

Here we have the newest version of the minimal OS used
in virt-test default tests, as well as the asset
file, used to download the iso conveniently with
tools/download_manager.py.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
run: Make JeOS 20 the default guest os in the runner

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
guest os: Remove JeOS 17

There's a limited hosting space, so it is necessary to
delete the older versions of JeOS available.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tools: Introduce package_jeos.py

This is a script used to sparsify and compress a
freshly installed new JeOS image for upload. It contains
the procedure so it may help people wanting to customize
JeOS.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>