An access has an id, and its mounts name it (issue 153)

Ten definitions name each access by id; the path stays as the default an assignment may replace,
and the host side of every mount says ${access:<id>}. Resolved with no placement, every definition
names exactly the paths it named before (TestPlacedDirectoriesKeepTheirPaths, extended). On an
adopted machine the assignment now says `accesses: {<id>: <path>}` and the mount follows.

Needs the controller that knows an access id (mesh-controller #176); the running one refuses the
field at registration.

modules/nats/module.json

@@ -62,13 +62,14 @@
       "volumes": [
         "/var/lib/mesh-broker-nats:/data",
         "/var/lib/nats-module/conf:/etc/nats:ro",
-        "/var/lib/mesh-broker-tls:/tls:ro"
+        "${access:tls}:/tls:ro"
       ],
       "artifact": "server"
     }
   ],
   "accesses": [
     {
+      "id": "tls",
       "path": "/var/lib/mesh-broker-tls",
       "mode": "read"
     }