Improve NNThroughputBenchmark to allow non super user to use the tool

The NNThroughputBenchmark can only be used with hdfs user or any user with super user privileges since entering/exiting safemode is a privileged operation. However, when using super user, ACL checks are skipped. Hence it renders the tool to be useless when testing namenode performance along with authorization frameworks such as Apache Ranger / any other authorization frameworks.

An optional argument such as -nonSuperUser can be used to skip the statements such as entering / exiting safemode. This optional argument makes the tool useful for incorporating authorization frameworks into the performance estimation flows.