Refactor the backup method of SwiftBackupService

Export functionalities of backup to 3 new methods:
prepare_backup backup_chunk and finalize_backup. The original
backup function calls those functions. This allows other
drivers to combine them in a different logic.
e.g for the RBDDriver

Change-Id: I4f335bfcd03cd1da52a9e5c5c689d97581614af6