Change query for mysql cluster size

In case of mysql failed it shows different values for different nodes.
After this fix it ll show actual cluster size

Change-Id: I2fcb23418d6de3e03108a1c66a6ebb6ee5b64008
Closes-Bug: PROD-17416

galera/files/grafana_dashboards/mysql_prometheus.json

           "tableColumn": "",
           "targets": [
             {
-              "expr": "mysql_wsrep_cluster_size{host=\"$host\"}",
+              "expr": "min(mysql_wsrep_cluster_size)",
               "format": "time_series",
               "intervalFactor": 2,
               "legendFormat": "",
   "timezone": "browser",
   "title": "MySQL",
   "version": 33
-}
+}