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: I21ea71b539a6ec35670152e427a7ef6d28863e2e

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 envlist = py3,pep8
 
@@ -7,7 +7,7 @@ envlist = py3,pep8
 basepython = python3
 setenv = VIRTUAL_ENV={envdir}
 usedevelop = True
-whitelist_externals = find
+allowlist_externals = find
 deps =
   -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
   -r{toxinidir}/requirements.txt
@@ -25,7 +25,7 @@ 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
 
 [testenv:debug]
 commands = oslo_debug_helper {posargs}
@@ -52,7 +52,7 @@ commands =
   {toxinidir}/tools/check_logging.sh {toxinidir}/manila
 
 [testenv:genconfig]
-whitelist_externals = bash
+allowlist_externals = bash
 commands =
   oslo-config-generator --config-file etc/oslo-config-generator/manila.conf
 
@@ -71,11 +71,11 @@ commands =
   sphinx-build -W -b html doc/source doc/build/html
   # Ignore D001 since we allow lines in excess of 79 characters.
   doc8 --ignore D001 --ignore-path .tox --ignore-path doc/build --ignore-path manila.egg-info -e .txt -e .rst -e .inc
-whitelist_externals = rm
+allowlist_externals = rm
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf
@@ -108,14 +108,14 @@ commands =
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
        pylint==2.3.1
-whitelist_externals = bash
+allowlist_externals = bash
 commands = bash ./tools/coding-checks.sh --pylint {posargs}
 
 [testenv:api-ref]
 # This environment is called from CI scripts to test and publish
 # the API Ref to docs.openstack.org.
 deps = {[testenv:docs]deps}
-whitelist_externals = rm
+allowlist_externals = rm
 commands =
   rm -rf api-ref/build
   python {toxinidir}/tools/validate-json-files.py {toxinidir}/api-ref/source/samples/