Simplify the steps to make changes to config.json (#5186)

This change introduces following simplified steps to follow 
during config migration.

```
 // Steps to move from version N to version N+1
 // 1. Add new struct serverConfigVN+1 in config-versions.go
 // 2. Set configCurrentVersion to "N+1"
 // 3. Set serverConfigCurrent to serverConfigVN+1
 // 4. Add new migration function (ex. func migrateVNToVN+1()) in config-migrate.go
 // 5. Call migrateVNToVN+1() from migrateConfig() in config-migrate.go
 // 6. Make changes in config-current_test.go for any test change
```

cmd/auth-rpc-server_test.go

@@ -28,7 +28,7 @@ func TestLogin(t *testing.T) {
 		t.Fatalf("Failed to create test config - %v", err)
 	}
 	defer os.RemoveAll(rootPath)
-	creds := serverConfig.GetCredential()
+	creds := globalServerConfig.GetCredential()
 	token, err := authenticateNode(creds.AccessKey, creds.SecretKey)
 	if err != nil {
 		t.Fatal(err)