Use python -m pip instead pip command.

It's recommand to use this way. With tox is not really necessary to do
it but it can be good to use it. See this article:

https://snarky.ca/why-you-should-use-python-m-pip/

For example python -m pip executes pip using the Python interpreter
you specified as python.

Change-Id: I99ba462eb3558bb45b8143cc2d20e72f8455033f

tox.ini

@@ -5,8 +5,6 @@ skipdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -U {opts} {packages}
-setenv = VIRTUAL_ENV={envdir}
 passenv =
     ANSIBLE_*
     CURL_CA_BUNDLE