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/auth-rpc-server_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.
@@ -22,7 +22,7 @@ import (
 )
 
 func TestLogin(t *testing.T) {
-	rootPath, err := newTestConfig("us-east-1")
+	rootPath, err := newTestConfig(globalMinioDefaultRegion)
 	if err != nil {
 		t.Fatalf("Failed to create test config - %v", err)
 	}
@@ -77,7 +77,7 @@ func TestLogin(t *testing.T) {
 		// Invalid password length
 		{
 			args: LoginRPCArgs{
-				Username: "minio",
+				Username: globalMinioDefaultOwnerID,
 				Password: "aaa",
 				Version:  Version,
 			},