The method Command#getNodeList in DiskBalancer was added in
HDFS-9545, but it's never used. We can improve that in the following aspects:
1.Change private to protected so that the subclass can use that method in the future.
2.Reuse the method Command#getNodeList and to construct a new method
like this List<DiskBalancerDataNode> getNodes(String listArg). This method can be used for getting multiple nodes in the future. For example, if we want to use hdfs diskbalancer -report -node or hdfs diskbalancer -plan with multiple specified nodes, that method can be used. Now these commands only support one node.