Fix missing tox in functional devstack-minimal based job

At the moment, the functional jobs derived from devstack-minimal does
not make sure tox is present, so the test fails.
This patch adds the role that ensure tox is present.

Change-Id: I9078ac2c4418d4a34cfb53dca8599deb3ec8d5f8

playbooks/functional/run.yaml

@@ -1,4 +1,7 @@
 - hosts: all
+  roles:
+    - ensure-tox
+
   tasks:
     - shell:
         cmd: |
@@ -45,6 +48,6 @@
 
           cd $IRONICCLIENT_DIR
           echo 'Running Functional Tests under Python3'
-          tox -e functionalpy3
+          {{ tox_executable }} -e functionalpy3
         executable: /bin/bash
         chdir: '/opt/stack/python-ironicclient'