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-parser_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.
@@ -81,7 +81,7 @@ func validateCredentialfields(t *testing.T, testNum int, expectedCredentials cre
 // A valid format of creadential should be of the following format.
 // Credential = accessKey + "/"+ scope
 // where scope = string.Join([]string{  currTime.Format(yyyymmdd),
-// 			"us-east-1",
+// 			globalMinioDefaultRegion,
 //               	"s3",
 //		        "aws4_request",
 //                       },"/")