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: I0968c6740ccb96d93527ba368eebe3e56e4e043f

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1.1
+minversion = 3.18.0
 skipsdist = True
 envlist = py{3}{,-mysql,-postgresql},functional,pep8
 ignore_basepython_conflict = True
@@ -23,7 +23,7 @@ commands =
    {toxinidir}/run-tests.sh {posargs}
    {toxinidir}/run-functional-tests.sh "{posargs}"
    aodh-config-generator
-whitelist_externals = bash
+allowlist_externals = bash
 
 [testenv:cover]
 commands =
@@ -48,7 +48,7 @@ setenv = PYTHONHASHSEED=0
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf