Add Heapsize variables for HDFS daemons

Currently there are no variables to set HDFS daemon heapsize differently. While still possible through adding the -Xmx to HDFS_DAEMON_OPTS, this is not intuitive for this relatively common setting.

YARN currently has these separate YARN_DAEMON_HEAPSIZE variables supported so it seems natural for HDFS too.

It also looks like HDFS use to have this for namenode with HADOOP_NAMENODE_INIT_HEAPSIZE

This JIRA is to have these configurations added/supported