Run only smoke tests in ipv6 job

'congress-tempest-ipv6-only' job is to verify
that congress is able to listen and run on IPv6
env. We do not need to run all the tests in this
job. Running only smoke tests should be fine.

This commit define the 'congress-ipv6' tox env which
include smoke test only as of now but it can be extended
to include more IPv6 tests if any.

Change-Id: Idbd4c6ea362487dd93330adddb26abc89387a726
Story: #2005477
Task: #35885

.zuul.yaml

@@ -69,7 +69,9 @@
     irrelevant-files: *base_irrelevant_files
     required-projects: *base_required_projects
     timeout: 6000
-    vars: *base_vars
+    vars:
+      << *base_vars
+      tox_envlist: congress-ipv6
 
 - job:
     name: congress-tempest-py2

tox.ini

@@ -83,6 +83,17 @@ deps = -r{toxinidir}/doc/requirements.txt
 commands = rm -rf doc/build doc/source/api
            sphinx-build -W -b html doc/source doc/build/html
 
+[testenv:congress-ipv6]
+envdir = .tox/tempest
+sitepackages = True
+setenv = {[tempestenv]setenv}
+deps = {[tempestenv]deps}
+# The regex below is used to select the congress smoke tests only. In future,
+# it can be extended to include more IPv6 related tests.
+commands =
+    find . -type f -name "*.pyc" -delete
+    tempest run --list-tests --regex '(^congress_tempest_plugin.*)(\[.*\bsmoke\b.*\])' {posargs}
+
 [testenv:releasenotes]
 basepython = python3
 deps = -r{toxinidir}/doc/requirements.txt