[FIX] product: allow multiple values for same attribute
[FIX] product: allow multiple values for same attribute

- Create a product attribute, e.g. 'Recommanded Age'
- Create several values for this attribute
- Try to associate more than attribute value to the same variant =>
  contraint which prevents to do it

We should allow this use case as long as the product attribute doesn't
create a variant.

opw-776344

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo