Change metrics for Cassandra Storage panel on the dashboard

New metrics have the same meaning and value as old ones.
Old metrics are not exported for some Cassandra versions.

Related-Bug: PROD-17651

Change-Id: If8a926a97c1d342b14381983a1b4c69aeac82275
diff --git a/opencontrail/files/grafana_dashboards/cassandra_prometheus.json b/opencontrail/files/grafana_dashboards/cassandra_prometheus.json
index b9354ea..9bad464 100644
--- a/opencontrail/files/grafana_dashboards/cassandra_prometheus.json
+++ b/opencontrail/files/grafana_dashboards/cassandra_prometheus.json
@@ -803,7 +803,7 @@
           "steppedLine": false,
           "targets": [
             {
-              "expr": "cassandra_db_StorageService_Load{host=~\"$host\"}",
+              "expr": "cassandra_metrics_Storage_Count{name=\"Load\",host=~\"$host\"}",
               "format": "time_series",
               "intervalFactor": 2,
               "legendFormat": "{{ host }}",
@@ -880,7 +880,7 @@
           "steppedLine": false,
           "targets": [
             {
-              "expr": "cassandra_db_StorageService_ExceptionCount{host=~\"$host\"}",
+              "expr": "cassandra_metrics_Storage_Count{name=\"Exceptions\",host=~\"$host\"}",
               "format": "time_series",
               "intervalFactor": 2,
               "legendFormat": "{{ host }}",