Fix: controller: panic local host if notified of own fencing

Previously, we attempted to reboot, but that would always fail because the
controller doesn't run as root, so it would fall back to exiting CRM_EX_FATAL.

Now, we exit CRM_EX_PANIC, to tell pacemakerd to panic the local host, which
is a better method of self-fencing.

clbz#5386