`swarm.authelia.url` defaulted to `https://<domain>` only when this host
ran the container, and to `null` otherwise — so the address a client is
given was a statement about co-location rather than about the swarm. A
swarm has one SSO provider; every hive addresses the same name and
resolution decides which address that reaches, exactly as
`swarm.otel.domain` already works.
The option stays nullable: "this swarm has no IdP" is still expressible,
it is just now something an operator states rather than something not
running the container produces. The Grafana fixture that exercised the
no-IdP refusal says it explicitly.
Closes#4536
4b6214305f