feat(alerting): changes default timerange to 15min

public/app/features/alerting/alert_def.ts

@@ -13,7 +13,7 @@ var alertQueryDef = new QueryPartDef({
     {name: "from", type: "string", options: ['1s', '10s', '1m', '5m', '10m', '15m', '1h', '24h', '48h']},
     {name: "to", type: "string", options: ['now']},
   ],
-  defaultParams: ['#A', '5m', 'now', 'avg']
+  defaultParams: ['#A', '15m', 'now', 'avg']
 });
 
 var conditionTypes = [