delete pods that have stopped
delete pods that have stopped

if `.poll()` notices that a pod has stopped, delete the pod

A `.delete_stopped_pods` flag is added to allow disabling this behavior if
deployments want to leave stopped pods around,
in case they are interested in inspecting the pod to diagnose the reason for stopping

yuvipanda/jupyterhub-kubernetes-spawner,jupyterhub/kubespawner