use  "stock definition supply" which overwrite default parameter in
purchase supply for the date included in "valid from" et "valid until"
if the value is not defined in "stock defintion supply", the default
parameter is used

This is much more convenient to understand stock value at a given point in time

/reviewed-on nexedi/erp5!705

/reviewed-on nexedi/erp5!671

1. accept a list of converison server
2. retry in case of network error

We need to be able to see the future without auto planned orders, else how are we going to update them?