ctdb-tools-ctdb: Generalise find_natgw() -> filter_nodemap_by_flags()

ctdb-tools-ctdb: Generalise find_natgw() -> filter_nodemap_by_flags()

Instead of just finding the first node that doesn't have any flags in
flag_mask set, change it into a function that filters a nodemap to
exclude nodes with the given flags.

This makes the NATGW code simpler but also provides a function that
can be used in other code.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>