Integrate realmdomains with IPA DNS
    
    Add an entry to realmdomains when a DNS zone is added to IPA.
    Delete the related entry from realmdomains when the DNS zone is deleted
    from IPA.
    
    Add _kerberos TXT record to DNS zone when a new realmdomain is added.
    Delete _kerberos TXT record from DNS zone when realmdomain is deleted.
    
    Add unit tests to cover new functionality.
    
    https://fedorahosted.org/freeipa/ticket/3544