[FIX] sale_dropshipping: set right dest_address_id

to populate dest_address_id on PO,
select partner_shipping_id on sale instead of partner's delivery contact.
in this way, shipping address will be correct in the two cases :
- standard case
- case when group_sale_delivery_address option is activated
Merge pull request #51 from rdeheele/7.0-sale_dropshipping-fix_shipping_address

[FIX] sale_dropshipping: set right dest_address_id