Add option to stateful manager to repeatedly poll job status.

In subsequent chagnesets this will enable message queue based manager to trigger a completion message and handle output collection without external intervention.
Add postprocessing state for complete jobs in stateful manager.

Doesn't actually do anything yet, but it will. Also replced string references to statuses with named constants in lwr.managers.status Also replaced some conditional imports with usages of six module.
Refactor remote staging actions toward being able to have postprocessing actions.

Make input to launch command a dictionary containing setup actions instead of a list. Move handling into manager so that processing staging actions can be done asynchornously like postprocessing actions will need to be.