Place kibana on manager

(for consistency, it's not strictly necessary)

docker-compose.logging.ops.yml

@@ -27,8 +27,7 @@ services:
   kibana:
     deploy:
       placement:
-        constraints:
-          - node.labels.type == external
+        constraints: [node.role == manager]
       labels:
         # router for basic auth based access (https)
         - "traefik.http.routers.kibana.rule=Host(`kibana.pdas.prism.eox.at`)"