scylla-jmx: Add dummy "compactionId" to compation info

To keep nodetool happy. Should maybe add actual Ids to compations.
Message-Id: <1482335118-9595-2-git-send-email-calle@scylladb.com>

src/main/java/org/apache/cassandra/db/compaction/CompactionManager.java

@@ -75,6 +75,7 @@ public class CompactionManager extends MetricsMBean implements CompactionManager
             result.put("keyspace", compaction.getString("ks"));
             result.put("columnfamily", compaction.getString("cf"));
             result.put("unit", compaction.getString("unit"));
+            result.put("compactionId", "<none>");
             results.add(result);
         }
         return results;