Whitelist mkdir as external in the tox.ini file

This will avoid the following warning message when running tox -e ical:

WARNING:test command found but not installed in testenv
  cmd: /usr/bin/mkdir
  env: .../irc-meetings/.tox/ical
Maybe forgot to specify a dependency?

Change-Id: I1e709b93337f71d369e0bcc9e37df196b1580232

tox.ini

@@ -7,6 +7,7 @@ skipsdist = True
 install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/test-requirements.txt
+whitelist_externals = mkdir
 
 [testenv:venv]
 commands = {posargs}