A callback is used to resolve the config at the time that
we set up the destination site structure (eg: when we get the first
ready-to-send message for it, and again after it has idled out).
This is used in the example configs to enable "OpportunisticInsecure"
TLS mode so that we can successfully delivery over TLS to the
default self-signed TLS certs on the sink.
Wez Furlong