LU-17108 nodemap: make map_mode available for default nm

LU-17108 nodemap: make map_mode available for default nm

The map_mode property lets control the way mapping is carried out. It
is already available on regular nodemaps, to decide whether uids, gids
and/or projids will be mapped.
On the default nodemap, where it is not possible to define mappings,
the map_mode property will be taken into account when trusted is 0 and
deny_unknown is 0. Unmapped IDs will be left unchanged.

Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I16a2f5cfda11a8435b56a00f3e97bdc70741c156