在namenode执行start-all.sh和stop-all.sh时候会通过ssh执行相关命令
cloudera网站：
When SSH is and is not Used
It is a good idea to use SSH for remote administration purposes (instead of rlogin, for example) but note that Hadoop itself and the related services do not use SSH for communication as a matter of course. Some scripts, and in particular the Hadoop start-all and stop-all scripts, do use SSH, but otherwise SSH is not used for communication among the following:
Datanode
Namenode
TaskTracker
JobTracker
/etc/init.d scripts (which start daemons locally)
参考：https://ccp.cloudera.com/display/CDHDOC/CDH3+Deployment+on+a+Cluster
