create mapred system directory if not exists + some code cleanup

This pull requests contains two commits - 

The first is minor code cleanup with no functional impact.  The second is to create the mapred system directory based first on mapreduce.jobtracker.system.dir value, and if not set the older mapred.system.dir property.

Fixes bug:
https://bugzilla.redhat.com/show_bug.cgi?id=996290