Split script-templates/magpie-hadoop-job into two files
Remove development note
Support 'hdfsonly' option to HADOOP_MODE, to make clear
requirement for setting it.

Fix script requirement for setting this w/ Hbase & Spark w/ HDFS
templates.
Clarify MAGPIE_ENVIRONMENT_VARIABLE_SCRIPT instructions
Support additional environment variables in MAGPIE_ENVIRONMENT_VARIABLE_SCRIPT