Add Backup/Restore Checksum Validation

Store the swift checksum for swift storage backups. The swift checksum is
the checksum of the concatenated segment checksums.  This is the checksum
that is stored with the DB backup record so we can verify that the checksum
on restore through a swift object HEAD call.

Adds restore from backup checksum validation at API and guestagent level.
* Verify checksum right before downloading swift file
* Verify the current swift backup file checksum matches the original backup
  checksum to prevent loading and restoring a tampered backup file.

Add swift checksum storage unit tests

Implements: blueprint verify-backup-checksum-on-restore
Change-Id: I2844cfb8a8b31f6ea7622f99eff3bea9455161e1