Organize configuration options

The number of configuration options available both in `~/.dask/config.yaml` as well as keywords to Scheduler/Worker/Client/LocalCluster ... are increasing rapidly. Unfortunately we don't handle these consistently. Some options you need to specify on the command line, others in config files, etc..

It would be useful if there was a consistent way to handle configuration options like these that both simplified things now and made it easy for developers to remain consistent in the future.