ensure-tox: use ensure-pip role

Use the ensure-pip role to ensure that `pip` is available.  If the
existing flag `tox_prefer_python2` is set, we pass along the flag to
ensure-pip to install Python 2 pip as well.

Story: #2007386
Task: #39308
Change-Id: I5053e315df8d2ea9892911800c88afcbe31cc05b

roles/ensure-tox/README.rst

@@ -20,3 +20,8 @@ a valid ``tox``.
 
    If tox is not detected, prefer to install tox inside Python 2
    instead of Python 3.
+
+   If set,
+   :zuul:rolevar:`ensure-pip.ensure_pip_from_packages_with_python2`
+   will be automatically set to `True` to enable a Python 2
+   installation of `pip`.

roles/ensure-tox/tasks/main.yaml

@@ -1,3 +1,9 @@
+- name: Install pip
+  include_role:
+    name: ensure-pip
+  vars:
+    ensure_pip_from_packages_with_python2: '{{ tox_prefer_python2 }}'
+
 - name: Ensure tox is installed
   shell: |
     set -euo pipefail

test-playbooks/ensure-tox.yaml

@@ -40,6 +40,7 @@
       pip:
         name: tox
         virtualenv: "{{ ansible_user_dir }}/tox-venv"
+        virtualenv_command: "{{ ensure_pip_virtualenv_command }}"
     - name: Run ensure-tox pointing to an already installed tox
       include_role:
         name: ensure-tox