k8s nfs backup: add rsync staging path and per-run staging cleanup

- Add BACKUP_STAGING_PATH + RSYNC_BIN for staging before archive
- Add cleanup_run_staging to EXIT trap to remove staging dir
- Extend test harness for staging scenarios

k8s/examples/automated-nfs-backup.env.example

@@ -5,6 +5,8 @@ KUBECTL_BIN=kubectl
 KUBE_CONTEXT=
 WORKLOAD_KINDS=deployment,statefulset,replicaset,replicationcontroller
 EXCLUDED_NAMESPACES=kube-system,kube-public,kube-node-lease,default
+BACKUP_STAGING_PATH=
+RSYNC_BIN=rsync
 ARCHIVE_PREFIX=nfs-backup
 ARCHIVE_TS_FORMAT=%Y%m%d_%H%M%S
 SEVENZ_METHOD=lzma2