Fix missing virtualenv in molecule tests

centos-8: Setup test-python task fails with:
Failed to find required executable virtualenv.
This was dropped recently in the base image, so we need
to handle this ourselves. We also need to make sure
tox is available in test-python so that we can invoke
molecule.

Change-Id: I200d64103bee540076551fd026cc8f4c21b40411

zuul.d/playbooks/pre.yml

@@ -11,12 +11,22 @@
         path: "{{ ansible_user_dir }}/zuul-output/logs"
         state: directory
 
+    - name: Ensure pip is available
+      include_role:
+        name: ensure-pip
+
     - name: Setup test-python
       pip:
         requirements: "{{ tripleo_ipa_project_path }}/molecule-requirements.txt"
         virtualenv: "{{ ansible_user_dir }}/test-python"
+        virtualenv_command: "{{ ensure_pip_virtualenv_command }}"
         virtualenv_site_packages: true
 
+    - name: Make sure tox is installed in test-python virtualenv
+      pip:
+        name: tox
+        virtualenv: "{{ ansible_user_dir }}/test-python"
+
     - name: Display test-python virtualenv package versions
       shell: |-
         . {{ ansible_user_dir }}/test-python/bin/activate