[ONOS-7350] Add ISSU HA/rollback test scenario

Change-Id: I8c1952480cd429d40c77db2e66d16ace27948148
diff --git a/core/net/src/main/java/org/onosproject/upgrade/impl/UpgradeManager.java b/core/net/src/main/java/org/onosproject/upgrade/impl/UpgradeManager.java
index ab1b1c3..218a357 100644
--- a/core/net/src/main/java/org/onosproject/upgrade/impl/UpgradeManager.java
+++ b/core/net/src/main/java/org/onosproject/upgrade/impl/UpgradeManager.java
@@ -385,6 +385,7 @@
                         .filter(id -> clusterService.getVersion(id).equals(upgrade.target()))
                         .collect(Collectors.toSet());
                 if (upgradedNodes.contains(event.subject().id())) {
+                    log.warn("Upgrade failure detected: rolling back upgrade");
                     rollback();
                 }
             }