Fix tox4 error

tox.ini started failing with tox4 which had some
incompatible changes. One of them is to allow
external command to run by adding them into allowlist_externals
and make passenv in new lines.

Fixing tox.ini for tox4 changes.

Change-Id: I0492630b17a7355a96abd44398b650fad9ec565f

tox.ini

@@ -1,7 +1,6 @@
 [tox]
 minversion = 2.0
 envlist = py37,pep8
-skipsdist = True
 ignore_basepython_conflict = True
 
 [testenv]
@@ -11,7 +10,13 @@ install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          PYTHONDONTWRITEBYTECODE = 1
          PYTHONWARNINGS=default::DeprecationWarning
-passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
+passenv =
+  http_proxy
+  HTTP_PROXY
+  https_proxy
+  HTTPS_PROXY
+  no_proxy
+  NO_PROXY
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
@@ -20,7 +25,7 @@ commands =
     rm -f .testrepository/times.dbm
     find . -type f -name "*.pyc" -delete
     stestr run --slowest {posargs}
-whitelist_externals =
+Allowlist_externals =
     rm
     find
 
@@ -51,7 +56,7 @@ commands =
   sphinx-build -E -W --keep-going -b html doc/source doc/build/html
 
 [testenv:releasenotes]
-whitelist_externals =
+Allowlist_externals =
   rm
 commands =
   rm -rf releasenotes/build