Permit to use existing LoadBalancer IP addresses

We must define DNS entries before deploying the stack to make Let's Encrypt HTTP challenge working:

1. reserve an IP address in the cloud provider
2. deploy the stack with the ingress controller.service.loadBalancerIP set to preallocated IP address.

We could add a vars.ini configuration like:

[general]
ingressController=nginx
ingressNamespace=ingress-nginx
ingressClassName=laboite-nginx
ingressLoadBalancerIP=X.X.X.X

As a workaround, we actually edit the ingress-nginx-values.yaml to set the value before running deploy.