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/object-api-getobject_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.
@@ -182,7 +182,7 @@ func testGetObject(obj ObjectLayer, instanceType string, t TestErrHandler) {
 // Wrapper for calling GetObject with permission denied expected
 func TestGetObjectPermissionDenied(t *testing.T) {
 	// Windows doesn't support Chmod under golang
-	if runtime.GOOS != "windows" {
+	if runtime.GOOS != globalWindowsOSName {
 		ExecObjectLayerDiskAlteredTest(t, testGetObjectPermissionDenied)
 	}
 }