[FIX] fleet: avoid function field computation order issues

The name of a vehicle is function field.
The field odometer on a vehicle has a a setter that creates an odometer.
The name of an odometer is a function field that uses the name of the vehicle.
If a vehicle is created with a value in odometer field, the odometer is created before the name is computed so the concatanation would fail.
Fixes #3468
[FIX] point_of_sale: rounding error in discount computing + removing duplicated code
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32