Test: cts-fencing: Bail out if fencer is running

If pacemaker-fenced is already running when cts-fencing starts,
cts-fencing will kill the existing fencer. Instead of allowing this to
happen, we should bail out and tell the user to stop the cluster.

Closes: T496

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>