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>