[FIX] product_extended: price from bom is build for product templates

As it sets the standard_price field, which is defined on product templates only: setting the standard price of a product variant will lead to set the standard price to all the variants of the product template.

There is therefore no meaning to compute and set the standard price of product variants according to the boms
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32