Output file name for mariabackup and xtrabackup providers

[mikegriffin]

Both versions appear to force stream format to mbstream or xbstream (tar no longer supported):

    else:
        args.append("--backup")
        if stream:
            args.append("--stream=xbstream")

Holland then keeps the originally configured stream method, ie tar by default, in the filename:

     if stream:
            if stream == "tar":
                archive_path = join(backup_directory, "backup.tar")
            elif stream == "xbstream":
                archive_path = join(backup_directory, "backup.xb")

It would be good for the mk-config command to not default to tar, and to change the output file name away from tar.