Drop unused parameter.
Rename Schedule -> TopologyScheduler.
whitespace
Start Schedule, which represents a topology schedule being built.
Schedule takes its topology details now.
Schedule#complete assigns a final schedule to the underlying cluster.
Show schedule declares all assigned executors.

..to a single slot per host.  Maybe not ideal, thinking about that still.
Prove trying to schedule on unknown machines fails harmlessly.
Schedule will not assign more than topology.getNumWorkers() slots.

Note it will still under-assign, and it pays no attention to any previous partial scheduling described by Cluster#getAssignmentById.