Customize Java Heap min/max settings for individual processes

The separate processes (master, regionserver, zookeeper, rest, etc.) all run using $JAVA_HEAP_MAX in 'bin/hbase'. However, there may be cases where we want the processes to run with different Java max heap sizes.

In 0.98.5, HBASE_(.*)_OPTS variables exist for these processes and can be used to add the Xms and Xmx tags, but because of how the default for JAVA_HEAP_MAX is set, it will always add '-Xmx1000m' to the final call to start up the master/regionserver/rest process, resulting in two different Java heap settings (e.g. -Xmx1000m and -Xmx8g is used when starting the master).

In 0.99.1, this default no longer exists so HBASE_(.*)_OPTS is usable, but it would be nice to have dedicated variables for each process.

This issue also exists in Hadoop HDFS (HDFS-7255).