Fix behaviour of gnt-node modify -C no

The current check on whether we require auto_promote or not is wrong, as
we check whether we will have exactly the correct number of master
candidates left. But it is fine if we have more (e.g. when CPS=10 and
mc_remaning=19) than the current number, and in that case we shouldn't
require auto promotion.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>