Specify {backend} for zaqar grenade CI
Specify {backend} for zaqar grenade CI

We missed to specify backend for Zaqar grenade CI. It will always
use mongo as the backend for different CI job which is wrong.

Change-Id: I04d1203b7db9c06b95a03bfdca0ead21e763352b

openstack-infra/project-config,openstack-infra/project-config