Every hive in a swarm occupies its own sub-domain of the swarm's, so
the hive domain is derivable rather than something each hive restates.
`services.hyperhive.swarm.domain` is new and nullable; the hive's own
`domain` keeps its existing required-ness and its existing assertion,
and gains a default of `<hiveName>.<swarm.domain>`.
Deliberately a default and not a rename: an alias would reinterpret the
domains hives have already deployed, while a default only fills in the
ones that never set one. Same reason there is no new assertion — the
existing message just names the derivation, so this adds a way to stop
failing rather than a way to fail.
