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.

Change-Id: I0ca0347bb6ebc5d8f5d22f708211e01221165262

tests/pre.yml

@@ -7,6 +7,11 @@
     - bindep
     - multi-node-firewall
   tasks:
+    # TODO(mnasiadka): Remove once infra merges virtualenv fixes
+    - name: Upgrade virtualenv package
+      command: python3 -m pip install -U virtualenv
+      become: True
+
     # We have had cases where the nodepool private IP address is not assigned,
     # which causes hard to diagnose errors later on. Catch it early.
     - name: Assert that the nodepool private IPv4 address is assigned