gitlab: introduce CI jobs testing git master & distro libvirt

The java build needs to validate two axis

 - A variety of libvirt versions
 - A variety of java versions

We get coverage for both these axis by running a build against the
distro provided libvirt packages. All that is then missing is a build
against the latest libvirt git master, which only needs to be run on
a single distro, for which Fedora 32 is picked. CentOS 8 is unable to
run tests, since it lacks ant-junit packages.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>