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: I533fe72cc3a5e1900d8f795582dd63d1f0aab70a

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.5.0
+minversion = 3.18.0
 envlist = py3,pep8,eslint,karma
 skipsdist = True
 
@@ -13,7 +13,7 @@ setenv =
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-whitelist_externals = npm
+allowlist_externals = npm
                       bash
 commands =
    python manage.py test octavia_dashboard --settings=octavia_dashboard.tests.settings
@@ -32,7 +32,7 @@ commands = npm install
            npm test
 
 [testenv:docs]
-whitelist_externals = rm
+allowlist_externals = rm
 deps =
     -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
     -r{toxinidir}/requirements.txt
@@ -43,7 +43,7 @@ commands =
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
   rm
 commands =
@@ -52,7 +52,7 @@ commands =
   make -C doc/build/pdf
 
 [testenv:releasenotes]
-whitelist_externals = rm
+allowlist_externals = rm
 deps = {[testenv:docs]deps}
 commands =
     rm -rf releasenotes/build