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/storage-rpc-server_test.go

@@ -38,7 +38,7 @@ func createTestStorageServer(t *testing.T) *testStorageRPCServer {
 		t.Fatalf("unable initialize config file, %s", err)
 	}
 
-	serverCred := serverConfig.GetCredential()
+	serverCred := globalServerConfig.GetCredential()
 	token, err := authenticateNode(serverCred.AccessKey, serverCred.SecretKey)
 	if err != nil {
 		t.Fatalf("unable for JWT to generate token, %s", err)