[FIX] Replacing date_order with date_confirm

Actually, procurements are planned using the date_order field from the order.
If you create your quotation one week ago and then valid your order, all dates (stock moves and picking) will be planned from the date_order that is not updated on confirmation.
It should take the confirmation date (date_confirm) of the sale order.

See [1] for more details
[1] https://bugs.launchpad.net/ocb-addons/+bug/1260778