Follow-up ticket for the multi-director setup

[bbockelm]

This ticket is the follow-up for items purposely deferred from #2023 to prevent that PR from becoming even more complicated. Each deferred item will be left as a comment within the issue.

(Labeling this as critical for v7.16 so none of the items linger)

[bbockelm]

TODO item:

In server_utils/director_discovery.go, we calculate the advertise URL (using Director.AdvertiseUrl and then falling back to Server.ExternalWebUrl if unset) on demand. It'd be better to move the calculation to config/config.go so the configuration parsing tools know this default value.

[bbockelm]

TODO item:

In director/director_advertise_test.go, we have several near-identical mock HTTP servers. To reduce code duplication (and avoid potential future bugs), refactor this into a more standalone function.