Glue for stock_virtual_reservation and sale

* we want to work with the virtual available as we cannot delivery what
is already outgoing
* call action_assign even if the state is already assigned, because it
may change the quantity

So we can activate the release only on some routes, while leaving
other routes work as normal (eg. MTO). Still have to figure out
if the ordered available to promise quantity is correct in this case.

'partner_id' is not required as the method _get_stock_move_values takes
the 'partner_id' from the 'group'. In fact, it makes the tests fail when
some other non-identified OCA addon is installed.