Change how NATGW is configured to allow special nodes that do not have

Change how NATGW is configured to allow special nodes that do not have
network connectivity outside of the cluster to still be able to
participate in a natgw group.
These nodes can not become natgw master since they lack external network
connectivity.

These nodes are configured just the same way as for any other node with
NATGW, with the following two exceptions :
* we do NOT set CTDB_NATGW_PUBLIC_IFACE at all on these nodes.
  since these ndoes lack external network we should not check the interface
  for link.
* we must set CTDB_NATGW_SLAVE_ONLY=yes to flag that this is a node that
  can not become natgw master.