CI: Install tox

tox will be removed from the base image. Install it before that happens.

This change is made in a simple way that can be easily backported.

Change-Id: I2a24c87aa44c1e03ba76a2fb0bd71caf54eb4211
(cherry picked from commit a81413f)

Author: markgoddard

tests/playbooks/pre.yml

@@ -15,3 +15,8 @@
       file:
         path: "{{ kolla_build_logs_dir }}"
         state: directory
+
+    - name: Ensure tox is installed
+      pip:
+        name: tox
+      become: true