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: I5792c86745b875d074f670d92fe78a84a907bd74

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.18.0
 skipsdist = True
 envlist = pep8,py,tht
 requires =
@@ -21,7 +21,7 @@ setenv =
 commands = {posargs}
 
 [testenv:pep8]
-whitelist_externals =
+allowlist_externals =
     bash
 commands =
     python ./tools/process-templates.py
@@ -67,7 +67,7 @@ commands = python ./tools/process-templates.py -c
 [testenv:releasenotes]
 deps = -c{env:TOX_CONSTRAINTS_FILE:https://releases.openstack.org/constraints/upper/master}
        -r{toxinidir}/doc/requirements.txt
-whitelist_externals =
+allowlist_externals =
     bash
 commands = bash -c tools/releasenotes_tox.sh
 
@@ -80,7 +80,7 @@ commands =
            python ./tripleo_heat_templates/environment_generator.py sample-env-generator/
 
 [testenv:genroledata]
-whitelist_externals =
+allowlist_externals =
     bash
 commands =
     bash -c tools/roles-data-generate-samples.sh
@@ -103,7 +103,7 @@ passenv =
     ANSIBLE_*
 deps =
   -r{toxinidir}/test-ansible-requirements.txt
-whitelist_externals =
+allowlist_externals =
     bash
 commands_pre =
     pip install -q bindep