Cleanup tox.ini

We remove install_command because the value set here is basically the
default but with one small difference. It doesn't run using `python -m
pip` which means we could potentially find a global pip install which
installs to the wrong python by default. Using python -m pip as the tox
default does should ensure it consistently finds the venv pip.

Additionally we stop allowlisting all external commands to make problems
like this more apparent.

Change-Id: I75a6bd7985fce66b3e8a4cb81320b22d71f6bc4a

tox.ini

@@ -26,8 +26,8 @@ setenv =
     PYTHONWARNINGS=default::DeprecationWarning,ignore::DeprecationWarning:distutils,ignore::DeprecationWarning:site
 passenv = OS_STDOUT_CAPTURE OS_STDERR_CAPTURE OS_TEST_TIMEOUT OS_TEST_LOCK_PATH TEMPEST_CONFIG TEMPEST_CONFIG_DIR http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY ZUUL_CACHE_DIR REQUIREMENTS_PIP_LOCATION GENERATE_TEMPEST_PLUGIN_LIST
 usedevelop = True
-install_command = pip install {opts} {packages}
-allowlist_externals = *
+allowlist_externals =
+    find
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt