ensure-tox: save tox_executable fact

If we install our own local version of tox, we should set a fact
with the tox_executable so other tasks can leverage it.

Change-Id: If6895bbb898261e88c0e3083d21210209f79995f

roles/ensure-tox/defaults/main.yml

@@ -0,0 +1 @@
+tox_executable: tox

roles/ensure-tox/tasks/main.yaml

@@ -14,5 +14,11 @@
   register: result
   changed_when: "'Successfully installed' in result.stdout"
 
+- name: Set tox_executable fact
+  set_fact:
+    tox_executable: "{{ ansible_user_dir }}/.local/bin/tox"
+    cacheable: true
+  when: result is changed
+
 - name: Output tox version
-  command: tox --version
+  command: "{{ tox_executable }} --version"