travis: move to CentOS 8 docker images
    
    As the CentOS 8 Docker images is finally out, we can use it and drop the
    plethora of workarounds we had to implement to compile RHEL8 systemd on
    CentOS 7.
    
    Resolves: #1761519
    
    patch_name: 0219-travis-move-to-CentOS-8-docker-images.patch
    present_in_specfile: true
    location_in_specfile: 219
    squash_commits: true