Small fixes to get things working on Kubeflow

[jacobtomlinson]

• Correctly detects the current namespace when run within the Kubernetes cluster.
• Use dask-operator namespace for the operator service instead of kube-system.
• Document enabling Kubeflow users to create Dask CRD resources.
• Have the scheduler listen on all addresses to enable service to work.
• Plays nicely with the notebook Istio/Envoy egress by renaming ports (and maybe unblocks Add full Istio support #488 🎉).
• Doesn't port forward the dashboard if running in-cluster.