feat(info): display information about Dask worker threads (#165)

Following the recent change that makes the number of threads
configurable for Dask workers, this commit adds the updates the
`/api/info/` endpoint to include the two introduces parameters:

* `dask_cluster_default_single_worker_threads`: Defines the default
   number of threads per single Dask worker.
* `dask_cluster_max_single_worker_threads`: Sets an upper limit on
   the number of threads per single Dask worker.

Closes reanahub/reana#874