update fuction for MongoDB custom resource

[horis233]

@kgcarr

Currently, MongoDB operator only supports create and delete function. We need to support update function when users edit MongoDB custom resource.

[horis233]

@kgcarr I can provide an idea about how to implement the update function.

1. In the 1Q release, we ignore all the IsAlreadyExists error when creating the resources.

   err = r.client.Create(context.TODO(), obj)
   if err != nil && !errors.IsAlreadyExists(err) {
       return fmt.Errorf("could not Create resource: %v", err)
   }

2. In the 2Q, we and another check for the IsAlreadyExists, if the resource is already exist, you can choose to update the resource.

   err = r.client.Create(context.TODO(), obj)
   if err != nil && !errors.IsAlreadyExists(err) {
       return fmt.Errorf("could not Create resource: %v", err)
   }
       if errors.IsAlreadyExists(err) {
                   err = r.client.Update(context.TODO(), obj)
                    if err != nil {
                        return fmt.Errorf("could not Update resource: %v", err)
                    }
       }

I am sure if all the MongoDB resources are permitted to be updated in the day2 option. You may need to take a look at this.