Provide a {{ flow('flow-name') }} jinja lookup for templating a flow's process group ID

[dbkegley]

This would be useful for the ReportingTask definitions in .flowlib.yml so that we can create reporting tasks for each flow that is deployed. This would allow metrics collection on a per-dataflow basis. We would need to re-deploy the associated ReportingTasks anytime the flow was deployed, or update the PG id in the metrics controller

[dbkegley]

If we allowed creating top level input/output ports for a flow we could also provide a remote('flow-name', 'port-name', [in,out]) so that we could send flowfiles via RPG to different flows or different instances