Improve test_nginx.sh
    
    - use download-prg.infra.opensuse.org (faster)
    - check if one of the roles installs packages we don't have in the
      docker image yet
    - replace systemctl (which refues to work inside docker) with a symlink
      to /usr/bin/true to avoid useless error messages
    - fix filename of subroles in grep command
    - run salt non-quiet so that we see error messages