records: use SOA mname instead of rname

[jdef]

fixes #501

[gpaul]

Using the mname instead of the rname is definitely correct. Would you mind adding a test to cover the failure we reported in #501 to this PR?

[jdef]

@gpaul I did a small bit of refactoring in a separate commit from the unit test I added. The refactoring helped to keep the test small, and more cleanly separates state loading from record generation (as it should be). When this PR lands, I'd like to keep the commit history vs. squashing it.

[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

Should I write up a separate ticket for this, or just fix as part of this one? @gpaul

[jdef]

answered my own question re: the above: opened issue #504 to track additional SOA mname rules

[gpaul]

LGTM, nice one