[ADD] product: Adding reference of Sellers on Product Variants.

Task: https://www.odoo.com/web#id=9402&view_type=form&model=project.task&action=327
Pad: https://pad.odoo.com/p/openerp-project.task-TAPAKZ5X7F

Description: This functionality will allow to add the reference of Suppliers to Product variants. Not modifying present functionality to take Supplier reference on Product.