[FIX] pos_discount: discount product

Only the products available in the POS must be set in the field
'discount_product_id'. Inspired from 8cd0127af3

opw:668163

addons/pos_discount/discount.py

@@ -14,7 +14,7 @@ class pos_config(osv.osv):
     _columns = {
         'iface_discount': fields.boolean('Order Discounts', help='Allow the cashier to give discounts on the whole order.'),
         'discount_pc': fields.float('Discount Percentage', help='The default discount percentage'),
-        'discount_product_id': fields.many2one('product.product','Discount Product', help='The product used to model the discount'),
+        'discount_product_id': fields.many2one('product.product', 'Discount Product', domain="[('available_in_pos', '=', True)]", help='The product used to model the discount'),
     }
     _defaults = {
         'iface_discount': True,