FIX(grafana): set default value for MinIO dashboard variable

- Set scrape_jobs default to 'minio'
- Hide variable selector (only one option)

grafana/dashboards/minio.json

@@ -3585,13 +3585,17 @@
   "templating": {
     "list": [
       {
-        "current": {},
+        "current": {
+          "selected": true,
+          "text": "minio",
+          "value": "minio"
+        },
         "datasource": {
           "type": "prometheus",
           "uid": "PBFA97CFB590B2093"
         },
         "definition": "label_values(minio_cluster_nodes_online_total, job)",
-        "hide": 0,
+        "hide": 2,
         "includeAll": false,
         "multi": false,
         "name": "scrape_jobs",