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: I5639fe73fe8ca4ae0e47eae0fb0b0f3c46c9ca19

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.5.0
+minversion = 3.18.0
 envlist = py3,pep8
 skipsdist = True
 ignore_basepython_conflict = True
@@ -7,7 +7,7 @@ ignore_basepython_conflict = True
 [testenv]
 basepython = python3
 usedevelop = True
-whitelist_externals =
+allowlist_externals =
     find
     rm
 install_command = pip install {opts} {packages}
@@ -30,7 +30,7 @@ commands = flake8
            # Run security linter
            {[testenv:bandit]commands}
            {toxinidir}/tools/coding-checks.sh --pylint {posargs}
-whitelist_externals =
+allowlist_externals =
   sh
   find
   bash
@@ -42,7 +42,7 @@ commands = {posargs}
 setenv =
   {[testenv]setenv}
   PYTHON=coverage run --source octaviaclient --parallel-mode
-whitelist_externals =
+allowlist_externals =
   find
 commands =
   find octaviaclient -type f -name "*.pyc" -delete
@@ -63,7 +63,7 @@ commands =
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
   rm
 commands =
@@ -139,7 +139,7 @@ deps =
 [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'