SingleCluster setup document needs to be updated

The following document needs an update:
https://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-common/SingleCluster.html

We need to set mapreduce.application.classpath, without that we cannot launch MR jobs using yarn.

 CLASSPATH for MR applications. A comma-separated list of CLASSPATH entries. If mapreduce.application.framework is set then this must specify the appropriate classpath for that archive, and the name of the archive must be present in the classpath. If mapreduce.app-submission.cross-platform is false, platform-specific environment vairable expansion syntax would be used to construct the default CLASSPATH entries.

So, add the default value for the tarball download setup.