Threat quantity_decreased equally for all sellables

Most places using quantity_decreased on stoq were changing it for all sellables,
even for services and products without storable, so, lets adjust all places to
do the same, otherwise we will not be able to trust quantity_decreased data.

There's a sql patch together to fix any mess those different apis probably made.

Change-Id: I435e1cdc4efe2ae74d30d045c9f9e6b5f3209960