Support different groups of workers for spawner and runner

[seankelly]

Right now, the logic for determining which workers to use for the spawner and runner is very simple. It would be very nice to have an easy way to create separate pools for them. My current thought would be to use a property set for the worker and key off that. Another option would be to allow listing them all in the configuration. That is less useful when using a YAML file an dynamically generating the number of workers.

[seankelly]

Looking through the source more, I now realize this is possible if runners are all latent workers and spawners are non-latent workers. I still think it would be useful to have additional options around this, but this is at least a reasonable start.