set default-ttl for zones, too

templates/zonefile.db.j2

 $ORIGIN {{ item }}.
-$TTL 60
+$TTL {{ powerdns.config["default-ttl"] }}
 @	SOA	{{ powerdns.zones[item].SOA }}
 {% macro generate_records(records, scope='@') %}
 {% for k, v in records.items() %}