Changed minversion in tox to 3.18.0

The patch bumps min version of tox to 3.18.0 python 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: I945f20d99bc4449fed5d6510d823b1375b2af22f

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 3.1
+minversion = 3.18.0
 # python runtimes: https://governance.openstack.org/tc/reference/project-testing-interface.html#tested-runtimes
 envlist = py3,pep8
 skipsdist = True
@@ -30,7 +30,7 @@ commands =
     stestr run {posargs}
     stestr slowest
 
-whitelist_externals =
+allowlist_externals =
   bash
   find
 passenv = *_proxy *_PROXY
@@ -55,7 +55,7 @@ commands =
     python -m stestr run --serial {posargs}
     python -m stestr slowest
 
-whitelist_externals =
+allowlist_externals =
   find
   stestr
 
@@ -68,7 +68,7 @@ basepython=python3.6
 # Not reusing py36's env due to https://github.com/tox-dev/tox/issues/477
 # envdir = {toxworkdir}/py36
 commands = {[testenv:functional]commands}
-whitelist_externals = {[testenv:functional]whitelist_externals}
+allowlist_externals = {[testenv:functional]allowlist_externals}
 
 [testenv:functional-py38]
 usedevelop=True
@@ -79,7 +79,7 @@ basepython=python3.8
 # Not reusing py38's env due to https://github.com/tox-dev/tox/issues/477
 # envdir = {toxworkdir}/py38
 commands = {[testenv:functional]commands}
-whitelist_externals = {[testenv:functional]whitelist_externals}
+allowlist_externals = {[testenv:functional]allowlist_externals}
 
 [testenv:releasenotes]
 # Not reusing doc's env due to https://github.com/tox-dev/tox/issues/477
@@ -98,7 +98,7 @@ commands =
   rm -rf doc/build .autogenerated doc/source/api
   sphinx-build -W -b html doc/source doc/build/html
   rm -rf api-ref/build
-whitelist_externals = rm
+allowlist_externals = rm
 
 [testenv:pdf-docs]
 deps =
@@ -108,8 +108,8 @@ commands =
   sphinx-build -W -b latex doc/source doc/build/pdf
   make -C doc/build/pdf
   cp doc/build/pdf/doc-cinderlib.pdf doc/build/html
-whitelist_externals =
-  {[testenv:docs]whitelist_externals}
+allowlist_externals =
+  {[testenv:docs]allowlist_externals}
   make
   cp