core: services: beacon: fix broken ethernet domain name config

[ES-Alexander]

Fixes #2548

⚠️ UNTESTED ⚠️ Probably a good idea to make sure this works properly for both ethernet and USB-OTG connections before merging.

If it's possible for the domain_names variable to not exist or be an empty list then we should catch/handle that (except the AttributeError, and/or make sure to append the hostname if it's not there at the end, like is being done with the default).