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: I124d2c272c1864c84d306ce2541d0288d781f1be

tox.ini

@@ -18,7 +18,6 @@ setenv =
     ANSIBLE_RETRY_FILES_ENABLED=0
     ANSIBLE_STDOUT_CALLBACK=debug
     PY_COLORS=1
-    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
@@ -32,7 +31,6 @@ passenv =
     SSH_AUTH_SOCK
     SSL_CERT_FILE
     TERM
-install_command = pip install {opts} {packages}
 deps =
        -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt