RBF: Add check to have single valid argument to safemode command

Scenario:

Current behaviour for the dfsrouteradmin command is: First argument should be valid one. What ever value we give as the second argument, the command is successfull.

Examples:

hdfs dfsrouteradmin -safemode enter leave

hdfs dfsrouteradmin -safemode leave enter

hdfs dfsrouteradmin -safemode get jashfuesfhsk

hdfs dfsrouteradmin -safemode leave leave

With the above examples, command is successfull for the first argument.

Expected:

Add check to have single valid argument to the safemode command