Reduce frequency of voltha-system-tests to avoid reusing executors

Change-Id: I45a69f78667154cb5dd558b72be26466a795a792
diff --git a/jjb/voltha-e2e.yaml b/jjb/voltha-e2e.yaml
index 4ecccf3..7d26848 100644
--- a/jjb/voltha-e2e.yaml
+++ b/jjb/voltha-e2e.yaml
@@ -69,7 +69,7 @@
     triggers:
        - timed: |
                  TZ=America/Los_Angeles
-                 H/20 * * * *
+                 H/30 * * * *
 
 - job-template:
     id: 'voltha-patch-test'