Add periodic-stable entry to .zuul.yaml

This adds a periodic-stable entry to .zuul.yaml for
periodically running stable jobs from previous releases
so we can gauge their stability over time using OpenStack
Health.

Change-Id: I672ff9c699afb7968f1cf0039915d5ce742da34c
diff --git a/.zuul.yaml b/.zuul.yaml
index 62800aa..77755ed 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -196,3 +196,8 @@
       jobs:
         - patrole-admin
         - patrole-member
+    periodic-stable:
+      jobs:
+        - patrole-member-rocky
+        - patrole-member-queens
+        - patrole-member-pike