Rename negative_ttl into default_ttl

[ulvida]

Hi! When I proposed the so called "dynamic" zones management (I think we should rather say raw zones and yaml zones, for instance), I did a mistake naming negative_ttl variable:

ansible-role-bind9/templates/bind/zones/db.template.j2

Line 24 in 88122fa

$TTL {{ zone.negative_ttl|default('3600') }} ; 1 hour

In fact, this value defines the $TTL BIND macro, which defines the default ttl for Ressource Records that don't have a specific TTL defined.
I also detailed a little bit the README doc.

[t2d]

Thank you for your PR. The addition to the README will make it much more easy to understand. I'm not sure the renaming of negative_ttl to default_ttl is really necessary. Negative TTL seems to be the technically correct term.

[ulvida]

FYI: we have a bigger PR in the pipe, that we are first testing ourselves with our DNS.

[t2d]

cool, thanks!