Adding hdfs user to stats group

This is so that hdfs user may write to the /var/log/kraken directory.
I am doing this with an exec because I don't want to start
managing the hdfs user in puppet just for this.  It is currently
added by the cdh4 packages.

Change-Id: Ifdf1ce4deb6adcc3a6b67c3a47e3a16e9f3fa121