Update the minversion parameter.

Update the minversion parameter to use the python -m pip to install
python packages:

https://tox.readthedocs.io/en/latest/changelog.html#id185

It's recommend to use this. Remove the useless install_command
parameter.

Change-Id: I25a43f463dbfac59a193774d6f90b0ddb73f5268

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.0
+minversion = 3.2.0
 envlist = py37,validate,pep8,bashate,docs
 skipdist = True
 ignore_basepython_conflict=true
@@ -9,7 +9,6 @@ usedevelop=True
 passenv=
   ZUUL_CACHE_DIR
   HOME
-install_command = pip install -U {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
    PYTHONUNBUFFERED=1