[FIX] sale_management: No discount displayed in option products

[FIRST ISSUE]

Steps to reproduce the bug:

- Create a pricelist PL with 50% reduction on all products.
- Create a SO with PL
- Add an optional product P

Bug:

 The price is correct for P  but the discount displayed was 0%.

[SECOND ISSUE]

Steps to reproduce the bug:

- Add the option line to the order

Bug:

A new line was create for P in the SO with the correct price but the discount was not displayed

PS: The discount field on the model sale.order.option has been implemented to add a new discount
to the displayed price unit of the option. But it has not been implemented to show the discount computed with
the pricelist.

opw:2186470

closes odoo/odoo#44703

Signed-off-by: Simon Goffin (sig) <sig@openerp.com>

addons/sale_management/models/sale_order.py

@@ -173,14 +173,15 @@ class SaleOrderOption(models.Model):
         if not self.product_id:
             return
         product = self.product_id.with_context(lang=self.order_id.partner_id.lang)
-        self.price_unit = product.list_price
         self.name = product.get_product_multiline_description_sale()
         self.uom_id = self.uom_id or product.uom_id
-        pricelist = self.order_id.pricelist_id
-        if pricelist and product:
-            partner_id = self.order_id.partner_id.id
-            self.price_unit = pricelist.with_context(uom=self.uom_id.id).get_product_price(product, self.quantity, partner_id)
         domain = {'uom_id': [('category_id', '=', self.product_id.uom_id.category_id.id)]}
+        # To compute the dicount a so line is created in cache
+        values = self._get_values_to_add_to_order()
+        new_sol = self.env['sale.order.line'].new(values)
+        new_sol._onchange_discount()
+        self.discount = new_sol.discount
+        self.price_unit = new_sol._get_display_price(product)
         return {'domain': domain}
 
     @api.multi