Upgrade virtualenv in pre
Upgrade virtualenv in pre

Since virtualenv 20.0 (amongst other changes) six version >1.12.0 is required.
This change adds upgrade of virtualenv and six in pre - to be reverted once
infra CentOS images are sorted out.

Depends-On: https://review.opendev.org/707377
Change-Id: I75249055989309d0afdfc29c894beb6b1fcd1994

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