[-] BO : Fixed bug #PSCFV-8895 - Cart rules query problem

Performance optimisation for slow query to get combinaison of cart
rules. Issues PSCFV-8895.
Replace left join with union clause in query.
Merge branch '1.6' of https://github.com/CIMEOS/PrestaShop into 1.6

Conflicts:
	classes/CartRule.php