Clean jobs after switching to wsgi by default in master

Remove the non-voting modwsgi job from master, now useless
(the base job uses wsgi).
Add two scenario test jobs against the stable branches to catch
regressions.

Story: 2001991
Task: 19617
Depends-On: https://review.openstack.org/567430
Change-Id: I72c51e5adf3ac3a88c8d641c8d85f5da6a5994b9

.zuul.yaml

@@ -4,9 +4,9 @@
         - openstack-tox-pylint:
             voting: false
         - sahara-tests-scenario
-        - sahara-tests-scenario-modwsgi:
-            voting: false
         - sahara-tests-tempest
+        - sahara-tests-scenario-queens
+        - sahara-tests-scenario-pike
     gate:
       jobs:
         - sahara-tests-scenario
@@ -102,6 +102,16 @@
       - ^releasenotes/.*$
       - ^sahara_tempest_plugin/.*$
 
+- job:
+    name: sahara-tests-scenario-queens
+    parent: sahara-tests-scenario
+    override-checkout: stable/queens
+
+- job:
+    name: sahara-tests-scenario-pike
+    parent: sahara-tests-scenario
+    override-checkout: stable/pike
+
 - job:
     name: sahara-tests-scenario-modwsgi
     description: |