Add config to backup all/only-labeled volumes

Currently, only labelled volumes are being backupped. This is a sub-optimal default value as some use-cases might also have anonymous volumes that should still being persisted and backupped.

Solution:

• Add a config-value backup-labelled-only with false as default value
• when listing the volume ids for backup, use all volume ids if the setting is false, otherwise test for label as is currently the case.