Default thread count for k8s workers

[martindurant]

By default the worker should use as many threads as cores (which has a pre-configured default). A user shouldn't be forced to configure the number of threads unless they want it to differ from the number of cores. I think we want worker_threads to set allow_none=True, then fix the logic in the k8s backend to use worker_cores if worker_threads is None.

Posting from #413 (comment)

Currently, if you don't specify the worker threads, you get

The value of the 'worker_threads' trait of a KubeClusterConfig instance should not be less than 1, but a value of 0 was specified.

[jcrist]

Note this is a major regression added in #353, and should be fixed before the next release.