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/handler-utils.go

@@ -43,7 +43,7 @@ func parseLocationConstraint(r *http.Request) (location string, s3Error APIError
 	} // else for both err as nil or io.EOF
 	location = locationConstraint.Location
 	if location == "" {
-		location = serverConfig.GetRegion()
+		location = globalServerConfig.GetRegion()
 	}
 	return location, ErrNone
 }
@@ -51,7 +51,7 @@ func parseLocationConstraint(r *http.Request) (location string, s3Error APIError
 // Validates input location is same as configured region
 // of Minio server.
 func isValidLocation(location string) bool {
-	return serverConfig.GetRegion() == "" || serverConfig.GetRegion() == location
+	return globalServerConfig.GetRegion() == "" || globalServerConfig.GetRegion() == location
 }
 
 // Supported headers that needs to be extracted.