Add python3 functional gate

Add a python3 functional gate that is non-voting.

Co-Authored-By: Lucas H. Xu <hanxu@redhat.com>

Change-Id: I5ca75cd85fa469cdaa869baa71726498e9d11e0b
Signed-off-by: Charles Short <chucks@redhat.com>
               Lucas H. Xu <hanxu@redhat.com>

.zuul.d/rally-tox-functional.yaml

@@ -17,5 +17,7 @@
     vars:
       devstack_plugins:
         rally-openstack: https://opendev.org/openstack/rally-openstack
+      devstack_localrc:
+        USE_PYTHON3: true
       rally_tox_env: "functional"
     run: tests/ci/playbooks/run-rally-tox.yaml

tests/functional/test_cli_env.py

@@ -171,8 +171,9 @@ class EnvTestCase(unittest.TestCase):
             plugings = "tests/functional/extra/fake_dir/fake_plugin.py"
             rally("--plugin-paths %s env check" % plugings)
         except utils.RallyCliError as e:
-            self.assertIn("There is no OSClient plugin 'noneclient'",
-                          e.output)
+            self.assertIn(
+                "Plugin existing@openstack.check_health() method is broken",
+                e.output)
 
     def test_check_api_info_fail_3(self):
         rally = utils.Rally()

tox.ini

@@ -56,6 +56,7 @@ basepython = python3.5
 commands = oslo_debug_helper -t tests {posargs}
 
 [testenv:functional]
+basepython = python3
 sitepackages = True
 commands =
   find . -type f -name "*.pyc" -delete