[FIX] product: keep attribute on a variant with only one attribute

When adding informational attribute, with only one possible value, it used to be skipped.
Instead keep it and add it on every variant.

To avoid dropping and recreating product (and lose eventual customisations), the attributes with only one possible value are set on every product.
This makes sure that in following test, these are not considered in variants_inactive variable.

Fixes #3204
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64