Finish implementing the ability of a sim-group to have default output-URL

[Baguage]

Moved from redmine, ticket 6367
This would enable each individual simulation to NOT have an output URL in the execution request. Currently, the same output URL is being used for all simulations.

A simulation's output URL would be filled out from the group's default output URL when the simulation description was written to the simulation's working directory. That way, the run_simulation.py script does not have to do any additional work, and can remain unchanged.