The ruby build needs to validate two axis

 - A variety of libvirt versions
 - A variety of ruby 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 CentOS 8 is picked as a stable long life
base.

For unknown reasons  "rake package" fails on OpenSUSE Leap 15, but it
appears to be a bug in the old version of rake, so we skip that part
of the build.

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