[FIX] purchase: select suppliers of variant
[FIX] purchase: select suppliers of variant

When a PO is created from a procurement, the field product_id of the
supplierinfo is not taken into account when the possible vendors are
chosen. For example, let's take the following situation:
- A product with 2 variants (color White and Black), route MTO
- Two vendors are created, one for color white, the other for color
  black
- When a SO containing the 2 variants is created, only one PO for the
  same vendor is created for both products.

With this fix, 2 POs will be created, linking the appropriate vendor and
variant.

opw-657362

bplancher/odoo,ygol/odoo,microcom/odoo,ygol/odoo,storm-computers/odoo,dfang/odoo,ygol/odoo,sysadminmatmoz/OCB,optima-ict/odoo,bplancher/odoo,hip-odoo/odoo,bplancher/odoo,stephen144/odoo,dfang/odoo,storm-computers/odoo,stephen144/odoo,ygol/odoo,optima-ict/odoo,storm-computers/odoo,Elico-Corp/odoo_OCB,sysadminmatmoz/OCB,laslabs/odoo,ygol/odoo,optima-ict/odoo,stephen144/odoo,hip-odoo/odoo,stephen144/odoo,microcom/odoo,laslabs/odoo,hip-odoo/odoo,Elico-Corp/odoo_OCB,stephen144/odoo,ygol/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,optima-ict/odoo,bplancher/odoo,dfang/odoo,sysadminmatmoz/OCB,stephen144/odoo,dfang/odoo,sysadminmatmoz/OCB,ygol/odoo,laslabs/odoo,sysadminmatmoz/OCB,microcom/odoo,Elico-Corp/odoo_OCB,dfang/odoo,optima-ict/odoo,storm-computers/odoo,laslabs/odoo,storm-computers/odoo,hip-odoo/odoo,bplancher/odoo,microcom/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,microcom/odoo,optima-ict/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,microcom/odoo,sysadminmatmoz/OCB,laslabs/odoo,sysadminmatmoz/OCB,storm-computers/odoo