Tests: Do not delete index templates on bwc tests

In order to run the bwc tests there is no need to delete the
index template at the end of a test run which results
in recreation of those due to all the cluster state listener.

Relates elastic/x-pack-elasticsearch#2228

Original commit: elastic/x-pack-elasticsearch@702d1c61ed

qa/rolling-upgrade/src/test/java/org/elasticsearch/upgrades/WatchBackwardsCompatibilityIT.java

@@ -121,6 +121,11 @@ public class WatchBackwardsCompatibilityIT extends ESRestTestCase {
         return true;
     }
 
+    @Override
+    protected boolean preserveTemplatesUponCompletion() {
+        return true;
+    }
+
     @Override
     protected Settings restClientSettings() {
         String token = "Basic " + Base64.getEncoder()