Set delete propagation policy (to "foreground")

Kubernetes 1.6 adds a `PpropagationPolicy` to the delete options,
obsoleting the previous `OrphanDependents` boolean.

This change sets `PropagationPolicy=foreground`, keeping the legacy
`OrphanDependents=false` for older k8s servers.

Note that PropagationPolicy=background is broken in k8s 1.6 and (eg)
leaks pods from a deployment.  See kubernetes/kubernetes#44046 for
upstream discussion.