tox: Add rm to whitelist_externals

rm is used to remove times.dbm between tests of py27 and py35. It should
be in whitelist_externals to avoid the warning:

WARNING:test command found but not installed in testenv
  cmd: /bin/rm
  env: /home/oanson/projects/dragonflow-2/.tox/py27
Maybe you forgot to specify a dependency? See also the whitelist_externals
envconfig setting.

Change-Id: I3ebe98f6cc17dced84621c6f089b4f04bb1a2300
Closes-Bug: #1711152

tox.ini

@@ -13,8 +13,8 @@ usedevelop = True
 install_command = {toxinidir}/tools/tox_install.sh {env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-whitelist_externals = sh
-commands = rm -f .testrepository/times.dbm
+whitelist_externals = sh /bin/rm
+commands = /bin/rm -f .testrepository/times.dbm
   ostestr '{posargs}'
 
 [testenv:fullstack]