[FIX] stock : traceback pivot view for stock moves

Use case when it can happens (non deterministic):
- Enable product variant
- Have different stock moves with the product variant
- Click on the stock moves in the section reports
- Switch to pivot view
-> Traceback : can't find path of null

This happens in the js in the function find_path_in_tree,
this function try to find path for children object. There is
a condition root.children[i].path[l] === path[l] (2 strings
comparaison).

With product variant this condition can be overpass when it should
have been triggered. The product path string is represented as follow
[Ref]Name(variants) -> Sometime the variants in the string do not have
the same order and thus the condition is never true and the function
return null instead of the correct root.

This commit corrects the name_get function for product product
in order to always return the variants in the same order.

addons/product/models/product_attribute.py

@@ -76,7 +76,7 @@ class ProductAttributevalue(models.Model):
 
     @api.multi
     def _variant_name(self, variable_attributes):
-        return ", ".join([v.name for v in self if v.attribute_id in variable_attributes])
+        return ", ".join([v.name for v in self.sorted(key=lambda r: r.name) if v.attribute_id in variable_attributes])
 
 
 class ProductAttributePrice(models.Model):