Set LIVE_MIGRATE_BACK_AND_FORTH=True for grenade live-migration job
Set LIVE_MIGRATE_BACK_AND_FORTH=True for grenade live-migration job

This makes the grenade live-migration job run live-migration
on a single instance to one compute node and then back to the
other, which is interesting in a grenade job where we have
mixed computes.

Change-Id: I06eb67105a36d3770febd400465edc0ca711c0cc
Depends-On: I8da2b3bd0c08d9a3111d3531c346d06bd52cae7b

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