changed env vars
added check_hadoop_namenode_heap.pl
extended env var to support just HADOOP
split off check_hadoop_yarn_node_managers.pl from check_hadoop_yarn_resource_manager.pl
updated check_hadoop_yarn_node_managers.pl
split off check_hadoop_yarn_resource_manager_heap.pl from check_hadoop_yarn_resource_manager.pl
split off heap and node manager checks to separate plugins to reduce number of switches and simplify code