Add support for onos-2.6 in Jenkins pipelines

Change-Id: I2659741aa148bbb1e94f50a7f868658cadf66f11
diff --git a/jjb/OnosSystemTest/onos-test-freestyle.yaml b/jjb/OnosSystemTest/onos-test-freestyle.yaml
index 1f310a2..53d5662 100644
--- a/jjb/OnosSystemTest/onos-test-freestyle.yaml
+++ b/jjb/OnosSystemTest/onos-test-freestyle.yaml
@@ -20,6 +20,7 @@
       - '2.3'
       - '2.4'
       - '2.5'
+      - '2.6'
       - 'master'
       - 'manually'