ctdb-eventscripts: Clarify that CTDB_NATGW_DEFAULT_GATEWAY is optional

ctdb-eventscripts: Clarify that CTDB_NATGW_DEFAULT_GATEWAY is optional

This has been implied since the command to add the route has had
errors redirected to /dev/null.  If infrastucture (e.g. ADS, DNS) is
on the same network as CTDB_NATGW_PUBLIC_IP then no route is
necessary.

Signed-off-by: Martin Schwenke <martin@meltin.net>