Test: cts-exec: Bail out if executor is running

If pacemaker-execd (or pacemaker-remoted if the "-R" option was used) is
already running when cts-exec starts, cts-exec will kill the existing
executor daemon. Instead of allowing this to happen, we should bail out
and tell the user to stop the cluster.

Closes: T497

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