[IMP] product variant: removed override of fields_view_get and use a "one group or...

[IMP] product variant: removed override of fields_view_get and use a "one group or another"
behavior for product variant. Users are either in group_product_mono or
group_product_variant, allowing to tune the form view according to the
group the user belongs to.
product: added group_product_mono group
sale: added set_group_product_variant method that adds or remove the
group_product_mono according to the group_product_variant being unchecked
or checked.
product: updated form view accordingly

bzr revid: tde@openerp.com-20140121105046-zkbs778upjg0lpyr