Whitelist binary 'rm' in tox.ini

To solve the following issue when running 'tox -e docs' it is necessary
to whitelist the binary 'rm'.

---snip---
WARNING:test command found but not installed in testenv
  cmd: /usr/bin/rm
  env: /home/berendt/development/branches/add_doc8/mistral/.tox/docs
Maybe forgot to specify a dependency?
---snap---

Change-Id: I120f221b64038d86afa6611f0f9634bce7b6fc0e

tox.ini

@@ -12,6 +12,7 @@ deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
   python -m mistral.openstack.common.lockutils python setup.py test --slowest --testr-args='{posargs}'
+whitelist_externals = rm
 
 [testenv:pep8]
 commands = flake8 {posargs}