Acceptance Tests: support choosing specific distro and version

Acceptance Tests: support choosing specific distro and version

The tests based on the LinuxTest class give the test writer a ready to
use guest operating system, currently pinned to Fedora 31.

With this change, it's now possible to choose different distros and
versions, similar to how other tags and parameter can be set for the
target arch, accelerator, etc.

One of the reasons for this work, is that some development features
depend on updates on the guest side.  For instance the tests on
virtiofs_submounts.py, require newer kernels, and may benefit from
running, say on Fedora 34, without the need for a custom kernel.

Please notice that the pre-caching of the Fedora 31 images done during
the early stages of `make check-acceptance` (before the tests are
actually executed) are not expanded here to cover every new image
added.  But, the tests will download other needed images (and cache
them) during the first execution.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210414221457.1653745-4-crosa@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Signed-off-by: Cleber Rosa <crosa@redhat.com>