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/signature-v4-utils_test.go

@@ -1,5 +1,5 @@
 /*
- * Minio Cloud Storage, (C) 2015, 2016 Minio, Inc.
+ * Minio Cloud Storage, (C) 2015, 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.
@@ -82,8 +82,8 @@ func TestIsValidRegion(t *testing.T) {
 	}{
 
 		{"", "", false},
-		{"us-east-1", "", true},
-		{"us-east-1", "US", true},
+		{globalMinioDefaultRegion, "", true},
+		{globalMinioDefaultRegion, "US", true},
 		{"us-west-1", "US", false},
 		{"us-west-1", "us-west-1", true},
 		// "US" was old naming convention for 'us-east-1'.