Backups storage

Add a backups archive storage abstraction and do a little cleanup.
It's basically so that any consumers of the backup metadata will know how to reproduce the checksum from the file for comparison with the checksum in the metadata.  Within juju there's no need to make use of the format in the metadata since for a given version there's only 1 format we're using.  I suppose it could matter if we were looking at a backup from an earlier version.  However, I doubt that's a use case we will ever need to worry about, much less in the short term. :)