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/hyperhive.nix

@@ -170,7 +170,7 @@ in
     '';
   };
 
-  # `enableAllLocalDefaults` is declared in ./local-defaults.nix, with
+  # `singleHostSwarm` is declared in ./local-defaults.nix, with
   # the values it asserts. It is a deployment mode rather than a setting
   # this module's options read, so it lives with its consequences.