Fix offline backup (#537)

This PR fixes #535 
Fixed:
- [x] Fixed `ScaledownCronJob`  does not scale up workload to 1 replica properly.
- [x] Fixed wait until workload scaled down and pod force deletion for satefulset/daemonset.

Issue to fix:
- [x] backup count of repository crd get updated from second backup only,  when status sub-resource is enabled.
- [x] can not get snapshot.
Error:
```
kubectl get snapshots --all-namespaces
No resources found.
Error from server: no pod found for workload stash-demo
```

- [x] Update test
- [x] Update Snapshot doc to specify in which situation user will fail to list `Snapshot`

backup.go

@@ -87,8 +87,14 @@ func NewCmdBackup() *cobra.Command {
 						log.Fatal(err)
 					}
 
-					if replica > 1 || !util.HasOldReplicaAnnotation(kubeClient, opt.Namespace, opt.Workload) {
-						log.Infof("Skipping backup. Reason: Backup type offline and replica > 1")
+					if replica > 1 {
+						log.Infof("Skipping backup...\n" +
+							"Reason: Backup type offline and replica > 1\n" +
+							"Backup has taken by another replica or scheduled CronJob hasn't run yet.")
+					} else if !util.HasOldReplicaAnnotation(kubeClient, opt.Namespace, opt.Workload) {
+						log.Infof("Skipping backup...\n" +
+							"Reason: Backup type offline and workload does not have 'old-replica' annotation.\n" +
+							"Backup will be taken at next scheduled time.")
 					} else {
 						log.Infoln("Running backup once")
 						if err = ctrl.Backup(); err != nil {