[Backup/Storage] Improve configuration and enable S3
The storage optiosn for backup do not work:
- If anything else than
FileSystemStorage
is used, nolocation
option must be passed - Other options cannot be configured
- It is cumbersome to pass an explicit storage class name
Please align the configuratio nwith that of the main storage, but use the main storage configuration options as defaults if they are set, except for th ebucket name, which is always explicitly required.