Workflow step plugins

[gschueler]

Add plugin support for workflow steps.

Workflow Step plugins should appear in the GUI when modifying a workflow, and be supported by XML/Yaml serialization.

Plugins should be able to be developed for these types of behaviors:

• a step that will execute once in a workflow and be passed the set of Nodes to execute on: a "Workflow Step Plugin"
• a step that will execute on each node defined for the job: a "Workflow Node Step Plugin"
• a plugin that just needs to generate a command/script to be dispatched to the nodes for the job: a "Remote Script Node Step Plugin"

Plugins should be able to be developed in Java. In addition Node Step Plugins should be definable as script-based plugins.

Configuration properties for plugins should be definable in the metadata of the Java class or script-plugin's plugin.yaml.

The configuration properties should also be able to define a "scope" whereby configuration values in a Project's project.properties or Framework's framework.properties can be used.