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: I76cd33019be6d7e3dbc6e1d94e64ee1286255839

tox.ini

@@ -5,11 +5,9 @@ skipdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/test-requirements.txt
 whitelist_externals = bash
 setenv =
-  VIRTUAL_ENV={envdir}
   ANSIBLE_ACTION_PLUGINS={toxinidir}/molecule/roles.galaxy/config_template/action:{toxinidir}/molecule/ansible_plugins/action
   ANSIBLE_CALLBACK_PLUGINS={toxinidir}/molecule/ansible_plugins/callback
   ANSIBLE_FILTER_PLUGINS={toxinidir}/molecule/ansible_plugins/filter
@@ -21,7 +19,6 @@ setenv =
   ANSIBLE_RETRY_FILES_ENABLED=0
   ANSIBLE_STDOUT_CALLBACK=debug
   ANSIBLE_LOG_PATH={envlogdir}/ansible-execution.log
-  VIRTUAL_ENV={envdir}
   # pip: Avoid 2020-01-01 warnings: https://github.com/pypa/pip/issues/6207
   # paramiko CryptographyDeprecationWarning: https://github.com/ansible/ansible/issues/52598
   PYTHONWARNINGS=ignore:DEPRECATION::pip._internal.cli.base_command,ignore::UserWarning