Workflowstep-specific environment variables

For monitoring purposes it can be useful to be able to have access to the current FlowNode/Step via environment variables.

This could be done by only injecting the current FlowNode id as environment variable.

A more flexible approach would be a new extension point, similar to an EnvironmentContributor, but called with the current StepContext.

If this idea gets a general go-ahead I will present a prototype.

Cc svanoort, abayer

Previous discussion: https://groups.google.com/forum/#!topic/jenkinsci-dev/VBYvIv3S_r4