Stop configuring install_command in tox.

Currently, we are overriding 'install_command' to use 'pip'. This is
considered poor behavior and 'python -m pip' should be used instead:

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

It turns out that this is the the default value provided by tox:

https://tox.readthedocs.io/en/latest/config.html#conf-install_command

So we can remove the line and simply use the default value.

Change-Id: I50080434a5a11226300eb699fd31103c5e9f5e28

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.0
+minversion = 3.2.0
 envlist = py3,pep8,docs
 skipsdist = True
 # this allows tox to infer the base python from the environment name
@@ -9,9 +9,6 @@ ignore_basepython_conflict=true
 [testenv]
 usedevelop = True
 basepython = python3
-install_command = pip install -U {opts} {packages}
-setenv =
-   VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = stestr run --slowest {posargs}