mongodb: its data directory is placed, not stated

The mesh resolves it to <root>/mongodb/data — where the granted
databases already sit. The provider's own state, grants and the mesh's
plumbing stay stated.

modules/mongodb/module.json

@@ -64,7 +64,6 @@
     {
       "id": "data",
       "type": "directory",
-      "path": "/var/lib/mongodb/data",
       "mode": "0700"
     },
     {
@@ -86,7 +85,7 @@
         "27017"
       ],
       "volumes": [
-        "/var/lib/mongodb/data:/data/db",
+        "${dir:data}:/data/db",
         "/var/lib/mongodb/root.secret:/run/secrets/root:ro"
       ]
     },