scylla-jmx: Exit on unknown parameter rather than infinite-loop

Ran into this while trying to use ccm with not up-to-date scylla-jmx.

Symptoms:

  $ ccm start
  Error starting node node1

and empty ~/.ccm/scylla-3/node1/logs/system.log.jmx
Message-Id: <1468399926-3565-1-git-send-email-tgrabiec@scylladb.com>

scripts/scylla-jmx

@@ -82,6 +82,9 @@ do
       shift 1
       ;;
     *)
+      echo "Unknown parameter: $1"
+      print_help
+      exit 1
     esac
 done