Add new configuration option for LM+grenade job
Add new configuration option for LM+grenade job

In order to test whether live migration is backward compatible
we need to live migrate VM back and forth between two versions
of nova. This configuration option will allow to reuse existing
tests just by adding if condition in a method that invokes live
migration and validates outcome:

* If set to False, it will use existing behaviour
* If set to True, it will live migrate VM, validate whether it
succeded, then live migrate the same VM once again and again
validate the result

Depends-On: Icaeca404ec3e4b8f3cd489789fdac6117740ec43
Change-Id: I8da2b3bd0c08d9a3111d3531c346d06bd52cae7b

openstack-dev/devstack,richliu/devstack,noironetworks/devstack,noironetworks/devstack,richliu/devstack,openstack-dev/devstack,group-policy/devstack,nawawi/openstack,group-policy/devstack,dtroyer/devstack,dtroyer/devstack,nawawi/openstack