SellableItemSlave: resets discount when adds item.

The manager discount is for an item only. When we add the item, we need
to restore the manager who authorized a discount to None before adding
another item.

Change-Id: I14c1481a61cd034eff4ac6a1b2ba3fb2ca72b167
Fixes: http://bugs.async.com.br/show_bug.cgi?id=5555
Merge "SellableItemSlave: resets discount when adds item."