[FIX] mrp: produce too much

Usecase:
- Produce 1 product not tracked
- Produce 1 product not tracked
The total number of finished product is 3

It happens because _set_quantity_done will
add the quantity and not set it to the quantity
passed as argument. In produce wizard it pass
as argument the quantity of move line + the produced
quantity. It will result as twice the quantity of the
move line + the quantity producing