Redesign master/client/server role boundaries to allow multi-machine tests

[LionsPhil]

Plumage's early design did not have distinct Master or UI roles, which is why the Client is responsible for managing the polygraph processes involved in a run, including commanding a Server to co-operate.

Now that the Master role exists, it would be cleaner for some of plumage_run's aggregation and control behaviour to be moved to it, leaving Client to act a lot more like Server as a temporary resource acting as a single component of a larger run without needing knowledge of it.

Machine resources could then potentially be offered as pools of multiple clients and servers, to allow multi-machine tests that scale beyond the capabilities of a single machine.

This would also pull host variables, and defining of client-server pairings, back toward the Master, centralizing configuration of them and making them easier to manage.