Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 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: I1d52cacc72f33b37c606512a14ff9f94c561a53f

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 distribute = False
 envlist = py3,pep8
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 
 [testenv]
@@ -10,7 +10,7 @@ setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
          OS_STDERR_CAPTURE=1
 
-whitelist_externals = find
+allowlist_externals = find
 deps =
        -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/test-requirements.txt
@@ -34,7 +34,7 @@ usedevelop = True
 deps =
        -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/doc/requirements.txt
-whitelist_externals = rm
+allowlist_externals = rm
 commands =
   rm -rf doc/build
   sphinx-build -b html doc/source doc/build/html
@@ -42,7 +42,7 @@ commands =
 [testenv:pdf-docs]
 usedevelop = True
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build  -W -b latex doc/source doc/build/pdf
@@ -52,7 +52,7 @@ commands =
 deps =
        -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/doc/requirements.txt
-whitelist_externals = rm
+allowlist_externals = rm
 commands =
     rm -rf releasenotes/build
     sphinx-build -a -E -W -d releasenotes/build/doctrees \
@@ -69,7 +69,7 @@ commands =
   stestr run {posargs}
 
 [testenv:genconfig]
-whitelist_externals = bash
+allowlist_externals = bash
 commands =
   {envdir}/bin/python setup.py install
   {envdir}/bin/oslo-config-generator --config-file etc/oslo-config-generator/manilaclient.conf