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.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2999
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    Reviewed-by: Petr Cech <pcech@redhat.com>