[FIX] stock: stock user create lot

When a stock user want to create a lot an exception is raised saying
that he cannot modify a product.

This is caused by the field 'product_uom_id' on the model
'stock.production.lot' that is a related to 'product.product' and not
set as readonly in the 'stock.production.lot' view.

So by setting it as readonly, we don't set the uom to the server, and so
don't try to re-write it on the product

addons/stock/views/stock_production_lot_views.xml

@@ -28,7 +28,7 @@
                             <label for="product_qty"/>
                             <div class="o_row">
                                 <field name="product_qty"/>
-                                <field name="product_uom_id" groups="product.group_uom"/>
+                                <field name="product_uom_id" readonly="1" groups="product.group_uom"/>
                             </div>
                         <field name="ref"/>
                     </group>