[CRM]: Trace-back in opportunities kanban(add new column) group by customer and salesperson

[IMP]model: In method add_default_value if field is selection and value is not exist in selection Set None.
More Details
Task: https://openerp.my.openerp.com/web#id=9047&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.JtoM7M4nWSvYQHSA