[IMP] product: improve variant demo data

Activate variant by default with demo data: since we have demo data creating
variants, the database will be more consistent if the option is also activated
by default. It also allows to gain time when making a demo or testing feature
using them on a new database.

addons/product/data/product_demo.xml

@@ -1,6 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <odoo>
     <data noupdate="1">
+        <!-- We want to activate product variant by default for easier demoing. -->
+        <record id="base.group_user" model="res.groups">
+            <field name="implied_ids" eval="[(4, ref('product.group_product_variant'))]"/>
+        </record>
 
         <record id="product_category_2" model="product.category">
             <field name="parent_id" ref="product.product_category_all"/>