Fix createActorSystem not actually using the systemName parameter.

This meant all system names were "spark", which worked, but didn't
lead to the most intuitive log output.

This fixes createActorSystem to use the passed system name, and
refactors Master/Worker to encapsulate their system/actor names
instead of having the clients guess at them.

Note that the driver system name, "spark", is left as is, and is
still repeated a few times, but that seems like a separate issue.