Pull logs on nightking before destroying instances

A desirable feature would be to get access to the logs of stark & whitewalker machines before destroying them.

I can imagine the Nightking can pull those logs in parallel from all starks + ideally from whitewalkers as well, at the end of each experiment. Then perhaps we would need a way to rotate these sets of logs on the Nightking (e.g., keep only the logs from the most recent 10 sets of experiments).

Such a feature would aid in debugging. Presently, a typical workflow whenever something goes wrong (e.g., throughput for an experiment is 0) is: set `DEBUG=1` on the Nightking, redo the problematic experiment, log in all starks, and investigate what went on by looking through the logs there.