SSSDConfig: Fix saving of debug_level
    
    SSSDConfig internally handle debug_level as an integer.
    But in case of bitmask version of debug_level (>=16)
    it stored value as a decimal which is confusing
    e.g.
        debug_level = 8176
    vs.
        debug_level = 0x1ff0
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3410
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit fca26b76f23ee4457d6796b19892ed97362b6c8d)
    (cherry picked from commit 712f1cc9acb8b71e6906d7a168a045920d491d96)