Automatically use hostpath if pod is on same host as zfs pool.

This addresses ccremer#85. When storage class type is set to auto,
automatically create a hostpath volume when the scheduler selects the
specified node to run the pod, otherwise fallback to using NFS.

Note this only works when volumeBindingMode is set to
WaitForFirstConsumer in the storage class. Otherwise, when set to
Immediate, volumes will be pre-provisioned before the scheduler selects a
node for the pod consuming the volume, and options.SelectedNode will be
unset.

Force HostPath to be used preemptively instead of letting the scheduler
choose where to place the first pod.

Also add support for ReadWriteOncePod.