Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 python in order to
replace tox's whitelist_externals by allowlist_externals option:
https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: Iad89625e7514d1046e0340db96a73960a17023cf

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 envlist = pep8,py36,py38
 skipsdist = True
 ignore_basepython_conflict = True
@@ -14,7 +14,7 @@ setenv =
    LC_ALL=en_US.utf-8
    PYTHONWARNINGS=default::DeprecationWarning
 passenv = OS_STDOUT_CAPTURE OS_STDERR_CAPTURE OS_TEST_TIMEOUT OS_TEST_LOCK_PATH http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY
-whitelist_externals = find
+allowlist_externals = find
 deps =
     -c{env:UPPER_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
@@ -53,7 +53,7 @@ setenv = VIRTUAL_ENV={envdir}
          NOSE_COVER_HTML=1
          NOSE_COVER_HTML_DIR={toxinidir}/cover
          NOSE_WHERE=patrole_tempest_plugin/tests/unit
-whitelist_externals = nosetests
+allowlist_externals = nosetests
                       rm
 description=Runs unit test code coverage report
 
@@ -64,12 +64,12 @@ deps =
 commands =
   rm -rf doc/build doc/source/framework/code
   sphinx-build -W -b html doc/source doc/build/html
-whitelist_externals = rm
+allowlist_externals = rm
 description=Generate patrole docs
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
    make
 commands =
    sphinx-build -W -b latex doc/source doc/build/pdf
@@ -81,7 +81,7 @@ deps = {[testenv:docs]deps}
 commands =
   rm -rf releasenotes/build
   sphinx-build -a -E -W -d releasenotes/build/doctrees -b html releasenotes/source releasenotes/build/html
-whitelist_externals = rm
+allowlist_externals = rm
 description=Generate release notes
 
 [testenv:debug]