Migration of sale_automatic_workflow to 8.0

# Migration of sale_automatic_workflow to 8.0

* Added some tests
* Removed the dependency on `sale_payment_method`: we should be able to use automatic workflows even if we don't use the sale payment methods. We will need to reimplement the few features I removed in a third "link" module between `sale_automatic_workflow` and `sale_payment_method`.
* Use new API whenever possible
* Updated `pot`
Not sure if it is good or not to change it.
This method should become a `@api.multi` in the base module the day it is migrated because `picking` should be `self`:
```
@api.multi
def _create_invoice_from_picking(self, vals):
```
I am a bit scared to use a new-api signature that will anyway not be compatible with the base new api.