Pin tox to be less than 4

The dev environment used by some docs developers uses
an old version of tox, and does not support the
whitelist_externals entry required by tox 4.

The version of tox used by some other developers cannot
support tox.ini that contains both allowlist_externals and
whitelist_externals in the same file.

The fix for now is to stay with whitelist_externals and tox<4

Signed-off-by: Ron Stone <ronald.stone@windriver.com>
Change-Id: I015b50a027669c62fc066b78b882c2888d7194cd

.zuul.yaml

@@ -1,5 +1,7 @@
 ---
 - project:
+    vars:
+      ensure_tox_version: '<4'
     templates:
       - stx-api-ref-jobs
     check:

tox.ini

@@ -18,26 +18,23 @@ deps =
 commands =
   sphinx-build -a -E -W --keep-going -d doc/build/doctrees -t starlingx -t openstack -b html doc/source doc/build/html {posargs}
   bash htmlChecks.sh
-allowlist_externals = bash
+whitelist_externals = bash
                       htmlChecks.sh
-whitelist_externals = {[testenv:docs]allowlist_externals}
 
 [testenv:api-ref]
 deps = {[testenv:docs]deps}
 commands =
   rm -rf api-ref/build
   sphinx-build -W -b html -d api-ref/build/doctrees api-ref/source api-ref/build/html
-allowlist_externals = rm
-whitelist_externals = {[testenv:api-ref]allowlist_externals}
+whitelist_externals = rm
 
 [testenv:linters]
-allowlist_externals = bash
 commands =
   bash -c "find {toxinidir}                   \
             \( -name .tox -prune \)           \
             -o -type f -name '*.yaml'         \
             -print0 | xargs -0 yamllint"
-whitelist_externals = {[testenv:linters]allowlist_externals}
+whitelist_externals = bash
 
 [testenv:venv]
 commands = {posargs}
@@ -48,16 +45,14 @@ passenv=PWD
 commands =
        bash -c "echo Running in {env:PWD}"
        bash new-topic.sh {env:PWD}
-allowlist_externals = new-topic.sh
+whitelist_externals = new-topic.sh
                       bash
-whitelist_externals = {[testenv:newfile]allowlist_externals}
 
 [testenv:picks]
 commands =
        ./pickCompare.sh
-allowlist_externals = pickCompare.sh
+whitelist_externals = pickCompare.sh
                       bash
-whitelist_externals = {[testenv:picks]allowlist_externals}
 
 
 [testenv:linkcheck]