virttest.bootstrap: Add get_jeos_info() API

With the newest JeOS 20, people running earlier
versions of Fedora as a host will have trouble
with the fact that libvirt of those hosts will
not know anything about 'fedora.20' guests.

So let's use the latest JeOS only for people
running on Fedora 20, and leave the older JeOS 19
for people running on other hosts.

In order to accomplish this goal, add get_jeos_info(),
a function that will return JeOS information according
to the host OS info, and change references to the
JeOS variants and asset names to use that API.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>