Restore node-step functionality for Job References

[gschueler]

Job references became workflow-steps in Rundeck 1.5, which prevents some jobs defined in Rundeck 1.4 from working as expected.

The previous behavior was that each Job Reference executed once per-node in the workflow.

Restore the option to have that behavior, by adding a "Node step" flag to Job references, so if necessary they can be executed per-node from a top-level workflow.

[gschueler]

GUI error changing job ref node step error handler to workflow step. need to correctly discard/delete workflow objects if validation fails