[fleet] New API migration

- Task: https://www.odoo.com/web#id=10075&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.xFp6558VZvondGaA
@ara-odoo
As one parent record is created automatically for 'fleet.vehicle.cost'.so while creating its object from 'fleet.vehicle.cost' form view , we are displaying parent_id field.so onchange works properly.
but while creating 'fleet.vehicle.cost' line from 'fleet.vehicle.log.contract', we are not displaying parent_id.
Structure is like that: one parent record(parent_id=False) is automatically created and lines that are added will use that id as parent_id.
By writing onchange of parent_id, it gives integrity error as vehicle_id is not calculated due to parent_id=false.