ctdb: Call out to ctdb_natgw helper from 11.natgw

ctdb: Call out to ctdb_natgw helper from 11.natgw

To keep this commit comprehensible, 11.natgw and the CTDB CLI tool are
temporarily inconsistent.  The tool will be made consistent in a
subsequent commit.

ctdb_natgw_slave_only() is reimplemented to check for the option in
the appropriate line in $CTDB_NATGW_NODES.

Update unit tests and documentation.

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