deploy: rename enableAllLocalDefaults to deploy.singleHostSwarm

Same defect as the switch below it, one tier up: it sat at the TOP of
`services.hyperhive`, a namespace that is meant to be everything about
hyperhive rather than the settings of a single hive. Whether this box is
the whole deployment is as per-host as a decision gets.

The name follows mara's sentence for what it means — "everything in the
swarm is running on this host" — rather than naming its mechanism.
"Defaults" was doing no work: it is not a defaults toggle, it is a claim
about where the swarm lives, and the pair now reads as the containment it
already was, singleHostSwarm implying allSwarmServices plus this hive.

One site was a setter rather than a reference: module-eval's `allLocal`
fixture passes an attrset merged into `services.hyperhive`, so its key
carries the path and had to become `deploy.singleHostSwarm`. A rename by
bare identifier is right for the twelve prose mentions and wrong for
exactly this one, which is worth knowing before the next rename.

nix/host-modules/hive-gateway/options.nix

@@ -92,7 +92,7 @@ in
         DNS shouldn't have a stale `/etc/hosts` entry sticking
         around. Requires `services.hyperhive.domain` to be set.
 
-        `services.hyperhive.enableAllLocalDefaults` turns this on as
+        `services.hyperhive.deploy.singleHostSwarm` turns this on as
         part of saying "this box is the whole deployment": that mode
         means there is no real DNS for these names and the operator is
         browsing them from the host itself. Set it here explicitly to