tools-ctdb: Add and use function filter_nodemap_by_natgw_nodes()

tools-ctdb: Add and use function filter_nodemap_by_natgw_nodes()

Add another filter function, like the ones for capabilities and flags
to, for filtering by NAT gateway nodes.  This makes the main
natgw_list function more readable.

Note that this drops the early filtering of disconnected nodes, so
they will now be listed in a NAT gateway group.  This makes sense.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
(Imported from commit ba69742ccd822562ca2135d2466e09bf1216644b)