Changes

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>

gitlab: add CONTRIBUTING.rst file to indicate use of merge requests
With the introduction of automated CI pipelines, we are now ready to
switch to using merge requests for the project. With this switch we
longer wish to have patches sent to the mailing list, and thus the
git-publish config is removed.
Reviewed-by: Andrea Bolognani <abologna@redhat.com> Signed-off-by:
Daniel P. Berrangé <berrange@redhat.com>