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.

Depends-On: https://review.opendev.org/713386

Change-Id: I4181654c88554c81940f0d079cf1d64326cdec79

tests/run.yml

@@ -59,6 +59,12 @@
       when:
         - ansible_architecture == 'aarch64'
 
+    - name: Ensure tox is installed
+      pip:
+        name: tox
+      when: need_build_image
+      become: true
+
     # NOTE(yoctozepto): required to template template_overrides.j2 for Zuul
     - name: Include kolla Zuul vars if building new images
       include_vars: