ipa-sidgen: reduce log level to normal if domain SID is not available
    
    To support AD trust agents, we need to run sidgen and extdom plugins
    on every IPA master. Lack of working configuration, thus, is not a
    failure so reduce log level to normal as sidgen plugin will not
    be active if domain SID is missing but it can certainly be kept
    enabled.
    
    Part of https://fedorahosted.org/freeipa/ticket/4951
    
    Reviewed-By: Tomas Babej <tbabej@redhat.com>