Add support for NTS on the server

[Ziris85]

Hello! This PR is just to add two vars that will allow the chronyd daemon to serve as an NTS server on the default port of 4460. I tested this and found out that all the bits to support NTS were already present, it just needed the ability to add the variables in.

There's actually a number of NTS-related configs that can go into chrony.conf, but seeings that this projects goal is to be a "simple" NTP container, and that chrony can operate with defaults on everything else, I've only included vars for the two configs necessary to actually enable NTP - ntsservercert & ntsserverkey.

Let me know what you think!