Additional rules needed for SOA mname

[jdef]

There's another, related issue that we could solve as part of this ticket: if the SOA mname isn't part of the mesos-dns domain, then mesos-dns SHALL NOT report an A RR. We don't perform any checks for this (https://github.com/mesosphere/mesos-dns/blob/9d21ea6de38bd371e34bde196ceaee94cdac8a2c/records/generator.go#L478).

See the examples here: http://www.zytrax.com/books/dns/ch8/soa.html

We should either:
(a) validate that the SOA mname is within the mesos-dns domain, or else;
(b) NOT generate A RR's for an mname that is outside the scope of the mesos-dns domain

[jdef]

related: DNS glue entries https://serverfault.com/questions/309622/what-is-a-glue-record