variant.on_hand will not return infinity any longer if variant.on_demand is true. Clients of these methods will have to handle the combinations between on_hand and on_demand values
This makes the default value of variant.on_demand false in all environments and in tests.
Additionally, adapt VariantStock.on_demand test and product factory to this change (setting on_hand value in product factory so it's not out of stock by default).
