Ensure virtualenv is installed.

As part of the cleanup test node python spec [1], the
pip and virtualenv elements were removed from the
infra image builds. Therefore, it's the zuul job itself
the one which needs to make sure virtualenv is installed.

This patch installs virtualenv using the generic zuul
role ensurei-virtualenv.

[1] - https://docs.opendev.org/opendev/infra-specs/latest/specs/cleanup-test-node-python.html

Change-Id: Id20ee6bf069f53739f7a5ea7983edf2fc9b445d2

zuul.d/playbooks/pre.yml

@@ -6,6 +6,10 @@
         path: "{{ ansible_user_dir }}/zuul-output/logs"
         state: directory
 
+    - name: Ensure virtualenv is available
+      include_role:
+        name: ensure-virtualenv
+
     - name: Setup bindep
       pip:
         name: "bindep"