add logstash_forwarder job
change logstash-forwarder to allow/require job-specific log path configuration
Merge branch 'master' into add-logstash_forwarder
support for job/global logstash forwarding
support customizing elasticsearch environment and options
auto-calculate ES_HEAP_SIZE if not provided
simplify some default elasticsearch gc/jvm options
Merge branch 'elasticsearch-vars-opts' into release-6
Add blobs:

 * logstash/logstash-forwarder-1.4.0.rc1
Release 6

 * added logstash-forwarder support for job logs
 * added elasticsearch options/env customization support