title: Auto generating a purchase order when a product (with quantity 0) is ordered
body: <p>When the product has quantity on hand equal to zero, and customer orders this product, then, we need Odoo to generate a purchase order automatically for this product.</p>
  
  <p>Odoo is not generating the automatic PO. We have Odoo 8.</p>
  
  <p>As for an example, I created a test product. I ticked "Buy" check box, and unticked the"Make to order" check box. I added a vendor (has an address) on the product as follows:
  Supplier: Old Stocks -
  Delivery Lead Time: 0 -
  Minimal Quantity: 1.00 </p>
  
  <p>I also created a reordering rule for this product:
  Name: PO/8406724
  Product: [9781452555508] 0.0.0.0 Would Our Legacy Survive? 
  Warehouse: Amman Stock -
  Location: AmmWH/Stock -
  Minimum Quantity: 5.000 -
  Maximum Quantity: 20.000 -
  Quantity Multiple: 1.000</p>
  
  <p>I made a purchase order for this product, and transferred the delivery order. The quantity on hand became 4 (less than the min. quantity in the reordering rule). Run the schedulers from Warehouse -> Run Schedulers. However, the RfQ was not created.</p>
  
  <p>What should I do to get an RfQ auto-generated, when the there is sales order for a product with "Quantity on hand" equal to zero?</p>
  
  <p>Thanks you :)</p>