Improve DNS record data validation
Implement missing validators for DNS RR types so that we can capture
at least basic user errors. Additionally, a normalizer creating
a fully-qualified domain name has been implemented for several RRs
where name server may mis-interpret the domain name otherwise.
Unit tests exercising these new validators for the most common
RR types have been added. This patch also consolidates hard-coded
values in DNS test to one place.
https://fedorahosted.org/freeipa/ticket/1106