Use stestr == 2.6.0 for OSP<15 when using git
Use stestr == 2.6.0 for OSP<15 when using git

stestr no longer supports python 2.7

Change-Id: Id8f483eff10497235c818bc2dcf3be81d709559b

redhat-openstack/infrared,redhat-openstack/infrared,redhat-openstack/infrared