test: pass stsRegionalEndpoints as regional in STS (#3309)

Author: trivikr

configuration.sample

@@ -1,6 +1,5 @@
 {
   "region": "us-east-1",
-  "stsRegionalEndpoints": "regional",
   "s3": {
     "params": { "Bucket": "test-s3-bucket" }
   },

features/sts/step_definitions/sts.js

@@ -1,6 +1,8 @@
 module.exports = function() {
   this.Before('@sts', function (callback) {
-    this.service = new this.AWS.STS();
+    this.service = new this.AWS.STS({
+      stsRegionalEndpoints: "regional"
+    });
     callback();
   });