tox: Add find command to allowlist_externals list

whitelist_externals is deprecated now, use allowlist_externals
instead. see[0]
Add find command to allowlist_externals list to fix a warning.[1]

[0]: https://tox.readthedocs.io/en/latest/config.html
[1]: https://zuul.opendev.org/t/openstack/build/9ac7f205080343f29d7e2b519abbdb73/log/job-output.txt#945

Change-Id: I67e9c892a945d04eff59427a0161aafceb136560

tox.ini

 [tox]
-minversion = 3.1
+minversion = 3.18
 skipsdist = True
 envlist = py38,linters
 ignore_basepython_conflict = True
@@ -7,7 +7,7 @@ ignore_basepython_conflict = True
 [testenv]
 basepython = python3
 usedevelop = True
-whitelist_externals = find
+allowlist_externals = find
                       rm
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
@@ -59,7 +59,7 @@ commands =
   sphinx-build -a -E -W -d deploy-guide/build/doctrees --keep-going -b html deploy-guide/source deploy-guide/build/html
 
 [testenv:pdf-docs]
-whitelist_externals = make
+allowlist_externals = make
 deps = {[testenv:docs]deps}
 commands =
   sphinx-build -W --keep-going -b latex doc/source doc/build/pdf
@@ -81,7 +81,8 @@ deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/doc/requirements.txt
-whitelist_externals = bash
+allowlist_externals = bash
+                      find
 commands =
   {[testenv:pep8]commands}
   {[testenv:doc8]commands}