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: I4bac7893751ee1c0548b7864d26774fed4f46a99

tox.ini

@@ -1,6 +1,6 @@
 [tox]
 envlist = py39,py38,pep8
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 
 [testenv]
@@ -13,7 +13,7 @@ usedevelop = True
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-whitelist_externals = sh
+allowlist_externals = sh
 commands =
   stestr run {posargs}
 # there is also secret magic in stestr which lets you run in a fail only
@@ -29,7 +29,7 @@ setenv =
   OS_ROOTWRAP_CMD=sudo {envdir}/bin/neutron-rootwrap {envdir}/etc/neutron/rootwrap.conf
   OS_ROOTWRAP_DAEMON_CMD=sudo {envdir}/bin/neutron-rootwrap-daemon {envdir}/etc/neutron/rootwrap.conf
   OS_FAIL_ON_MISSING_DEPS=1
-whitelist_externals =
+allowlist_externals =
   sh
   cp
   sudo
@@ -51,7 +51,7 @@ setenv =
   OS_TEST_PATH=./neutron_vpnaas/tests/functional/openswan
   {[testenv:functional]setenv}
 deps = {[testenv:functional]deps}
-whitelist_externals = {[testenv:functional]whitelist_externals}
+allowlist_externals = {[testenv:functional]allowlist_externals}
 commands =
   {toxinidir}/tools/deploy_rootwrap.sh {toxinidir} {envdir}
   stestr run {posargs}
@@ -61,7 +61,7 @@ setenv =
   OS_TEST_PATH=./neutron_vpnaas/tests/functional/strongswan
   {[testenv:functional]setenv}
 deps = {[testenv:functional]deps}
-whitelist_externals = {[testenv:functional]whitelist_externals}
+allowlist_externals = {[testenv:functional]allowlist_externals}
 commands =
   {toxinidir}/tools/deploy_rootwrap.sh {toxinidir} {envdir}
   stestr run {posargs}
@@ -85,7 +85,7 @@ commands =
   neutron-db-manage --subproject neutron-vpnaas --database-connection sqlite:// check_migration
   {[testenv:genconfig]commands}
   {[testenv:genpolicy]commands}
-whitelist_externals = sh
+allowlist_externals = sh
 
 [testenv:pep8-dev]
 deps =
@@ -119,7 +119,7 @@ commands = sphinx-build -W -a -b html doc/source doc/build/html
 [testenv:pdf-docs]
 envdir = {toxworkdir}/docs
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf