ensure-tox: add prefer_python2

This patch enables the ability to set a value which prefers the
installation of tox within python2 over python3.

Change-Id: I29c9585aa9048c0e2855ec1eaf1f48041cfe46e2

roles/ensure-tox/README.rst

@@ -2,3 +2,18 @@ Ensure tox is installed
 
 If tox is not already installed, it will be installed via pip in the
 user install directory (i.e., "pip install --user").
+
+**Role Variables**
+
+.. zuul:rolevar:: tox_executable
+   :default: ``tox``
+
+   Optional path to point tox executable
+
+.. zuul:rolevar:: tox_prefer_python2
+   :default: ``true``
+
+   If tox is not detected, prefer to install tox inside Python 2 instead of
+   Python 3.  The default value is currently set to ``true`` for compatibility
+   reasons, it will be switched to false eventually.  It's best to set this to
+   ``false`` when using it.

roles/ensure-tox/defaults/main.yml

@@ -1 +1,2 @@
 tox_executable: tox
+tox_prefer_python2: true

roles/ensure-tox/tasks/main.yaml

@@ -2,11 +2,19 @@
   shell: |
     set -euo pipefail
 
+    {% if tox_prefer_python2 %}
     if command -v pip; then
       PIP=pip
     elif command -v pip3; then
       PIP=pip3
     fi
+    {% else %}
+    if command -v pip3; then
+      PIP=pip3
+    elif command -v pip; then
+      PIP=pip
+    fi
+    {% endif %}
 
     type tox || $PIP install --user tox
   args: