Refactor dacluster to start controller and engines separately #454
Description
From @minrk :
"""
You can start the controller and engines separately, which makes restarting engines easier (though they will get new IDs). I typically do it this way:
ipcontroller
ipcluster engines —engines=MPI -n 8
Then I can kill/restart the engine processes.
"""
We should refactor dacluster to use this pattern, so that dacluster restart simply kills the engine processes and then fires up a new set of them.