Sync stopping of workers on DRMAA Cluster
Sync stopping of workers on DRMAA Cluster

The DRMAA Cluster can close workers synchronously or asynchronously.
However the point of the `shutdown_distributed` function is that we want
to handle this synchronously. So just set the `sync` option to `True`.
That way we can be sure all workers have stopped on the DRMAA Cluster
before proceeding. The Local Cluster case below already syncs stopping
each worker. IOW now both cases have stopped all workers for sure before
leaving the `try...except...else` block.

nanshe-org/nanshe_workflow,DudLab/nanshe_workflow