Add constants for commonly used values. (#3588)

This is a consolidation effort, avoiding usage
of naked strings in codebase. Whenever possible
use constants which can be repurposed elsewhere.

This also fixes `goconst ./...` reported issues.

cmd/xl-v1-list-objects-heal_test.go

@@ -1,5 +1,5 @@
 /*
- * Minio Cloud Storage (C) 2016 Minio, Inc.
+ * Minio Cloud Storage (C) 2016, 2017 Minio, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,7 +25,7 @@ import (
 // TestListObjectsHeal - Tests ListObjectsHeal API for XL
 func TestListObjectsHeal(t *testing.T) {
 
-	rootPath, err := newTestConfig("us-east-1")
+	rootPath, err := newTestConfig(globalMinioDefaultRegion)
 	if err != nil {
 		t.Fatalf("Init Test config failed")
 	}