kolla-toolbox: use u-c while installing virtualenv
kolla-toolbox: use u-c while installing virtualenv

We need to be sure that we use exact same version every time. So fetch
upper-constraints.txt file earlier and use it while installing
virtualenv package.

Change-Id: I61711a878b2bda9f0d2e88966f13121dfcddfda7

openstack/kolla,stackforge/kolla,openstack/kolla,stackforge/kolla,stackforge/kolla