beah-srv: let systemd kill only the main process
    
    Bug: 908354
    task is left in running state after rhts-reboot on RHEL7
    
    At shutdown, systemd by default kills whole cgroup. So if it kills
    part of test first (for example sleep in rhts-reboot) then test can
    reach end of runtest.sh and beah will see this as "test finished".
    Then after reboot test won't resume running because master.get_done()
    will evaluate as True.
    
    Signed-off-by: Jan Stancek <jstancek@redhat.com>