ctdb-eventscripts: CTDB_NATGW_PUBLIC_* optional on slave-only nodes

ctdb-eventscripts: CTDB_NATGW_PUBLIC_* optional on slave-only nodes

Commit 4ee4925d416a86341bd76c11fa99ec9173682a1d forgot about
CTDB_NATGW_SLAVE_ONLY so it introduces an incorrect failure when this
is set, and CTDB_NATGW_PUBLIC_IFACE or CTDB_NATGW_PUBLIC_IP is unset.

Relax the sanity check to see if CTDB_NATGW_SLAVE_ONLY is set.

Update the documentation to explicitly state that
CTDB_NATGW_PUBLIC_IFACE and CTDB_NATGW_PUBLIC_IP are optional and
unused if CTDB_NATGW_SLAVE_ONLY is set.  It would be possible to
insist that CTDB_NATGW_PUBLIC_IFACE and CTDB_NATGW_PUBLIC_IFACE should
be unset in that case.  However, it is more reasonable to allow
consistent configuration across nodes except with some nodes
configured slave-only.

Add tests, update infrastructure and fix a thinko in the stub's
"natgwlist" implementation.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Autobuild-User(master): Martin Schwenke <martins@samba.org>
Autobuild-Date(master): Mon Apr 14 06:06:49 CEST 2014 on sn-devel-104