gitlab-ci: Introduce new 'integration_tests' pipeline stage

gitlab-ci: Introduce new 'integration_tests' pipeline stage

This stage will download build artifacts from both the libvirt and
libvirt-perl (multi-project CI) builds, install all them on the custom
runners and configures libvirt debug logging on the runners prior to
executing the actual test suite. In case of a failure, libvirt and
Avocado logs will be saved and published as pipeline artifacts.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>