IPA: Ignore dns_discovery_domain in server mode
    
    https://fedorahosted.org/sssd/ticket/2079
    
    If the dns_discovery_domain is set in the server mode, then the current
    failover code will use it to discover the AD servers as well. This patch
    resets the discovery domain unless the admin configured SRV resolution
    for IPA servers manually. In the case he did, we try to warn him that
    service discovery of AD servers will most likely fail.