Be able to diferentiate kube-apiserver loadbalancer listener port and pool member port

[faelis]

/kind feature

Describe the solution you'd like
When creating the loadbalancer in front of the kube-apiserver, it would be useful for us to be able to customise the port the pool member is listening to.

something like that :
kubectl --> openstack kube-apiserver load balancer on port 443 --> controlplane1 or controlplane2 or controlplane3 on port 6443

Anything else you would like to add:
N/A