Add max TTL time to D2

With the addition of being able to set the DDNS TTL with ddns-ttl-percent to some percentage of lease time, it might be a good idea to add a maximum possible TTL to the kea-ddns configuration. This should be an absolute time limit. For example, 3600 for one hour maximum TTL length. In this example, if Kea sent something higher than this, D2 would truncate the TTL to 3600. If the proposed setting is not present in D2 then there should be no change from current behavior which, I believe, is no limit to the length of the TTL.

RT21610