Re-order gate cleanup pip package removal

The packages are now removed in reverse dependency order. The previous
order would remove packages until a dependancy conflict would arise,
then would stop and the pip task would return 'changed' rather than
'failed'. See [1].

In addition, the pip installation of tox is removed.

[1] http://paste.openstack.org/show/789996/

Change-Id: I4f701ca5e7caa4d5ef182ca3eba523702bdae36a

zuul.d/playbooks/pre-gate-cleanup.yml

@@ -29,16 +29,17 @@
     - name: Remove known problem pip packages
       pip:
         name:
+          - virtualenv
+          - tox
           - appdirs
-          - backports
           - contextlib2
           - distlib
           - filelock
           - importlib_metadata
           - importlib_resources
           - six
-          - virtualenv
-          - zipp 
+          - zipp
+          - backports
         state: absent
 
     - name: Remove package blacklist for yum/dnf