ganeti-noded: add the --no-mlock option

While mlock on noded is definitely good in most situations, there are
some - namely my laptop - where it has no benefit, and uses precious
non-swappable memory. To avoid this we make it optional, with a new
--no-mlock option. Note that only the main node daemon and its http
children are affected: the powercycle node child still uses mlock, which
doesn't harm, since it's a short lived process happening just before
node reboot anyway. The manpage is updated.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Luca Bigliardi <shammash@google.com>