It turns out a minimal Fedora installation doesn't have tar by default.
Since we use tar when uploading the default image to glance, we need to
make sure it's installed. And since it might be useful for other very
generic tasks DevStack might do, just always install it (by putting it
in the general list of packages).

Change-Id: I4dae3b43a700d80b8752a5b846a6ce302fadb751