DEBUG: Add `debug` alias for debug_level
Our users constantly make the mistake of typing `debug = 9` in the
sssd.conf instead of `debug_level = 9` as would be correct. This
happens frequently-enough that we should just alias it rather than
continue to have people make mistakes.
Reviewed-by: Pavel Březina <firstname.lastname@example.org>
Reviewed-by: Petr Cech <email@example.com>