Arvados

AFAICT the "-" entries were added in #15003 with intent to indicate "ExternalURL is not needed" (as opposed to "...not configured yet"), but the config-load/check don't warn about missing entries anyway, and if/when we do add such warnings, we can arrange to not warn about the services that don't need external URLs. So all the "-" entries really seem to do is create an unnecessary mystery for a person editing a config file.

18287-external-url-defaults @ 0bfda57681195c431e3b6063577fdab23ff40cd5 -- developer-run-tests: #2736

This LGTM, thanks.