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: I9130a9202515ac7b88e7b6a99dbb6a756ac65895

tox.ini

@@ -7,8 +7,6 @@ ignore_basepython_conflict=true
 [testenv]
 basepython = python3
 usedevelop = True
-install_command = pip install -U {opts} {packages}
-setenv = VIRTUAL_ENV={envdir}
 deps = -c{toxinidir}/upper-constraints.txt
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
@@ -33,7 +31,6 @@ commands = update-requirements {posargs}
 
 [testenv:generate]
 # Generate needs an unconstrained install to get new dependencies
-install_command = pip install -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = generate-constraints {posargs}
@@ -85,7 +82,6 @@ commands = sphinx-build -W -b html doc/source doc/build/html
 [testenv:pip-install]
 recreate = True
 deps = .
-install_command = pip install {opts} {packages}
 commands = python {toxinidir}/tools/check-install.py
 
 [testenv:requirements-check]