[IMP] (website_)sale: Allow configuring variants creation on demand

Purpose
=======

Product attributes can be configured to create variants "on demand".
This means that product.templates having these attributes will only create product.product variant
when that combination is actually selected in a sale order/in the web shop.

Product attribute values are also improved to handle extra prices and exclusions
in all three modes ("never", "on demand", "always")

Specification
=============

Create variants on demand
- add a new mode to generate variants as they are added to the order/cart
  (needed for big # of combinations managed in the inventory and/or produced in mrp with specific boms)
- replace the boolean by a selection radio
  tooltip for this new field:
	Always:
	  Variants are generated straight away when
	  the attribute values are combined in the product configuration.
	Only when the product is added to a sales order:
	  Variants are generated once the combination is selected in the sales order.
	  This is advised when variants are customized on demand and manufactured as such in Odoo.
	Never:
	  Advised when variants are not needed in Odoo.
	  The selected values just show up in the description of sales order lines.
- ability to define attribute extra prices & exclude combinations in all variant modes
- if at least one attribute of the product in set to "on demand" mode, generate all the variants "on demand"