Bump tox minversion to 3.18.0

Since tox 3.18.0, the whitelist_externals option has been deprecated in
favor of the new allow_list_externals option[1].

This change bumps the minversion of tox so that we can replace
the deprecated option.

[1] https://github.com/tox-dev/tox/blob/master/docs/changelog.rst#v3180-2020-07-23

Change-Id: I39e2118e01d4ea87e782b7e3ba3956d5fa57f383
diff --git a/tox.ini b/tox.ini
index 86d81b1..a0a2885 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1
+minversion = 3.18.0
 envlist = pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -32,7 +32,7 @@
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source octavia_tempest_plugin --parallel-mode
-whitelist_externals =
+allowlist_externals =
   find
 commands =
   find octavia_tempest_plugin -type f -name "*.pyc" -delete
@@ -48,14 +48,14 @@
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
     -r{toxinidir}/doc/requirements.txt
-whitelist_externals = rm
+allowlist_externals = rm
 commands =
   rm -rf doc/build
   sphinx-build -W -b html doc/source doc/build/html
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
   rm
 commands =
@@ -105,7 +105,7 @@
   ./octavia_tempest_plugin/hacking
 
 [testenv:genconfig]
-whitelist_externals = mkdir
+allowlist_externals = mkdir
 commands =
          mkdir -p etc
          oslo-config-generator --output-file etc/octavia.tempest.conf.sample \
@@ -114,6 +114,6 @@
 [testenv:requirements]
 deps =
   -egit+https://opendev.org/openstack/requirements#egg=openstack-requirements
-whitelist_externals = sh
+allowlist_externals = sh
 commands =
     sh -c '{envdir}/src/openstack-requirements/playbooks/files/project-requirements-change.py --req {envdir}/src/openstack-requirements --local {toxinidir} master'