[FIX] crm: set partner correctly while converting lead to opportunity

- Task: https://www.odoo.com/web/#id=10133&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.cPUeuuC5qdmBykVI
Yes..If partner already exists, then on any of selection('create new customer' or 'link to existing customer') will link that opportunity to already exist partner, not new partner.