Changes to perform rolling restart of HDFS datanode

Refer #194. The changes are to perform rolling restart if HDFS datanode service.Also there are HDFS datanode related changes for #309, #305 & #183.    
` node[:bcpc][:hadoop][:zookeeper][:servers] ` currently stores hostname and node number. We can extend it to have the ` mgmt_ip ` as well.