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: I17ce1b72e7e9acb64b342a149b68ad31b79a2dff

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 2.0
+minversion = 3.18.0
 envlist = py36,py38,pep8,docs
 skipsdist = True
 ignore_basepython_conflict = True
@@ -24,7 +24,7 @@ commands =
     coverage html -d cover
     coverage xml -o cover/coverage.xml
     coverage report -m
-whitelist_externals = rm
+allowlist_externals = rm
 
 [testenv:cover]
 deps =
@@ -53,7 +53,7 @@ commands =
   bandit -r barbican -x tests -n5 -s B105
 
 [testenv:genconfig]
-whitelist_externals = bash
+allowlist_externals = bash
 envdir = {toxworkdir}/pep8
 commands =
   oslo-config-generator --config-file etc/oslo-config-generator/barbican.conf
@@ -82,11 +82,11 @@ deps =
 commands=
     rm -rf doc/build doc/build/doctrees
     sphinx-build -W -b html doc/source doc/build/html
-whitelist_externals = rm
+allowlist_externals = rm
 
 [testenv:pdf-docs]
 deps = {[testenv:docs]deps}
-whitelist_externals =
+allowlist_externals =
   make
 commands =
   sphinx-build -W -b latex doc/source doc/build/pdf
@@ -107,7 +107,7 @@ commands=
     {[testenv:docs]commands}
     {[testenv:api-guide]commands}
     {[testenv:releasenotes]commands}
-whitelist_externals = rm
+allowlist_externals = rm
 
 [testenv:functional]
 # This tox env is purely to make local test development easier