subtests.cfg.sample: Small adjustments to the test list

We're not supposed to install JeOS, unless on the libvirt
case, where we have to import the JeOS image so libvirt
can create a domain XML for it. So restrict tests with
JeOS to only those mentioned.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>