Add the "alloc_policy" attribute to node groups

This can be set at group creation time and via OpSetGroupParams. The default
is "preferred", and existing node groups from previous Ganeti version will
get the attribute set to this value.

Signed-off-by: Adeodato Simo <dato@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>