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)