Add dns_discovery_domain option
    
    The service discovery used to use the SSSD domain name to perform DNS
    queries. This is not an optimal solution, for example from the point of
    view of authconfig.
    
    This patch introduces a new option "dns_discovery_domain" that allows to set
    the domain part of a DNS SRV query. If this option is not set, the
    default behavior is to use the domain part of the machine's hostname.
    
    Fixes: #479