Remove SGs from non-existent nodes ports on Update

[EmilienM]

Current code assumes that if a Node has been removed from the list on UpdateLoadBalancer(), we don't care about it's security group. We could update applyNodeSecurityGroupIDForLB() to also lookup ports that have the SG tag and detach the SG from the ports that aren't belonging to the ports of the current list of Nodes.

https://github.com/kubernetes/cloud-provider-openstack/blob/04bd8e1343d630a0db3e9093dd71f59e477ee7ad/pkg/openstack/loadbalancer_sg.go#L72-L99