use ES6 arrow function instead of self var on getAggregators

public/app/plugins/datasource/opentsdb/query_ctrl.ts

@@ -38,9 +38,8 @@ export class OpenTsQueryCtrl extends QueryCtrl {
       this.target.downsampleFillPolicy = 'none';
     }
 
-    var self = this;
-    this.datasource.getAggregators().then(function(aggs) {
-      self.aggregators = aggs;
+    this.datasource.getAggregators().then((aggs) => {
+      this.aggregators = aggs;
     });
 
     // needs to be defined here as it is called from typeahead