Split ensuring tox is installed into a role

We do this in a few places, make it reusable.

Change-Id: Ied118508dc4b8e4cbfca88b5636730664dbe10b4

playbooks/tox/pre.yaml

@@ -1,4 +1,3 @@
 - hosts: all
-  tasks:
-    - name: Ensure tox is installed
-      shell: type tox || pip install --user tox
+  roles:
+    - ensure-tox

roles/ensure-tox/tasks/main.yaml

@@ -0,0 +1,2 @@
+- name: Ensure tox is installed
+  shell: type tox || pip install --user tox