Implement simple Gitlab-CI pipeline for MariaDB with RPM builds

As Travis-CI has stopped offering free testing for open source projects,
and they don't seem to have any plans to revert their new restrictions,
MariaDB no longer has a good CI system outside contributors could run
independently for basic validation before submitting Pull Requests.

Implement a simple Gitlab-CI pipeline that runs basic RPM builds on
one old, one less old and one very new distro release and then do some
basic tests on the RPM packages to validate they installed and the
server actually runs.