Add support for the DS record (with updated field names)

[baest]

Also updated tests to add a new DS record for a sub zone along with updates since an existing DS records is now handled Depends on octodns/octodns#1065

[ross]

This PR will need to hold off until octodns/octodns#1065 is merged, a new octoDNS release is made, AND we're ok with hard requiring that version to use octodns-powerdns.

For now that means #49 (comment) makes sense. If I had to guess i'd say this PR can be considered for merging ~6 months or so after octodns/octodns#1065 is merged and a release with it cut.

[baest]

I have an idea to handle this interim period with some version checking of octodns. Since if octodns/octodns#1065 is merged this will start failing.
We can undo the changes to tests/config/unit.tests.yaml and use old field names for now since that is backwards compatible. In __init__.py we can check the version of octodns and behave accordingly (I have a local patch). Problem is that we then violate the 100% test coverage since octodns can't be installed as both before and after #1065.

version_check.patch

[ross]

version_check.patch

This patch got me thinking about options for how to navigate our way out of the compatibility issues outlined in octodns/octodns#1065 (comment). I came up with #51 which should do the trick.

Instead of looking at octoDNS versions it instead looks directly at the functionality it's handling, namely whether DsValue.flags is a property. If it is then it uses old names, if not new.

Closing this as #51 will be the path forward.