Patches for allowing upgradeprovision to upgrade SD

[Matthieu Patou]

Hello Andrew and Nadya,

So following the morning talk we had with andrew regardings SD, I 
introduced a new control: recalculate_sd that is used in the descriptor 
module.
It is used by the update script to force the recalculation of the SD of 
an object as if the object was created (well at least it's the intent of 
it ...).

It seems to work pretty well as I've been able to upgrade 2 differents 
provision from 2 differents times.

The script is for the moment making the simplistic but quite ok 
assumption that nobody has changed the SD set by the provision.
This assumption has to be changed but it's to my mind very important to 
offer to people running alpha8 or older a possibility to update SD as 
the new acl module is already making a lot of checks (that didn't make 
my life too simple ...).

As it's a bit late I didn't take the time to merge with the change 
introduced by Andrew on the provision script this morning I'll try to do 
it tomorrow but andrew feel free to do it if my patches please you !

Matthieu.