[server, webui]: Add hostname value, optional support for external-dns

[lobis]

Closes #248

We have been trying external-dns to manage the dns records for our services with good results.

external-dns uses annotations in the services in order to work.

Getting the correct configuration with the current rucio charts was not straightforward and it required having duplicated config values (you don't want to hardcode the hostname in multiple places). This can be fixed by adding a configuration value named hostname which then can be referenced elsewhere.

The way to enable external-dns is via a disabled-by-default flag named useExternalDNS which adds the necessary annotations. While the current design which allows arbitrary annotations works in theory, it requires duplication of values.

I refactored the section related to the service annotations since now we have more than one source of annotations.