[IMP] sale: allow override sale_order_discount product

Add method '_get_discount_product' to allow to override the product used
by the sale_order_discount wizard.

closes odoo/odoo#153652

Signed-off-by: Jérémy Kersten <jke@odoo.com>

addons/sale/wizard/sale_order_discount.py

@@ -63,16 +63,21 @@ class SaleOrderDiscount(models.TransientModel):
 
         return vals
 
-    def _create_discount_lines(self):
-        """Create SOline(s) according to wizard configuration"""
+    def _get_discount_product(self):
+        """Return product.product used for discount line"""
         self.ensure_one()
-
         discount_product = self.company_id.sale_discount_product_id
         if not discount_product:
             self.company_id.sale_discount_product_id = self.env['product.product'].create(
                 self._prepare_discount_product_values()
             )
             discount_product = self.company_id.sale_discount_product_id
+        return discount_product
+
+    def _create_discount_lines(self):
+        """Create SOline(s) according to wizard configuration"""
+        self.ensure_one()
+        discount_product = self._get_discount_product()
 
         if self.discount_type == 'amount':
             vals_list = [