Lookup domains at startup
    
    To make sure that e.g. the short/NetBIOS domain name is available this
    patch make sure that the responders send a get_domains request to their
    backends at startup the collect the domain information or read it from
    the cache if the backend is offline.
    
    For completeness I added this to all responders even if they do not need
    the information at the moment.
    
    Fixes https://fedorahosted.org/sssd/ticket/1951