Optionally set jupyterhub pod as DaskCluster owner?

Over in https://github.com/pangeo-data/pangeo-binder/issues/143, we've noticed that some dask-gateway pods hang around. The root cause is likely something like https://github.com/ipython/ipykernel/issues/462. But even if that's fixed, there might be cases like a segfault in the client where we can't hope to inform other pods to close.

When deployed with jupyterhub, we might have a second option for cleaning up this pods. We could add the jupyterhub user pod as an owner of the `daskcluster` thing (pod? app?) created when a user does `gateway.new_cluster()`. Then when the jupyterhub user pod goes away then the dask-gateway resources are cleaned up too.

Does that seem OK to you (perhaps as an option exposed in the helm chart)? If so I'd love to work on it. It'd be a good way to dive into the internals a bit.