DNS install check: allow overlapping zone to be from the master itself
    
    When re-running `ipa-server-install --setup-dns` on already installed
    server, we do not get to the check of being already installed because
    DNS zone overlap forces us to fail earlier.
    
    Change exception returned for this case from check_zone_overlap() to
    return structured information that allows to understand whether we are
    finding a conflict with ourselves.
    
    Use the returned information to only fail DNS check at this point if DNS
    zone overlap is generated by a different name server than ourselves.
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>