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.    
@cbaenziger, we should be able to provide the node holding the lock which will help admins. But need to be aware that the lock was not acquired since other fully functional nodes were holding the lock.

@amithkanand, why not if that will help the admin.