Update gitlab-ci.yml with new syntax
With newer versions of GitLab, we've introduced a few new features that would allow us to simplify our gitlab-ci.yaml file considerably.
One of them is: extends
which will allow us to reuse a template and add more to it based on the specific requirement
Another one is: parallel
which will remove the need to repeat some of the jobs we currently have defined.
Finally, we should review our pipeline with: only/if syntax to not start jobs unless we need to, for example SLES packages.