Add support for deletion protection for Load Balancers

[mikkeloscar]

ALBs and NLBs have a feature to set deletion protection: https://docs.aws.amazon.com/elasticloadbalancing/latest/network/network-load-balancers.html#deletion-protection to avoid users from accidentally deleting the load balancer when clicking around in the AWS Console.

We already have a feature to protect CloudFormation stack termination: https://github.com/zalando-incubator/kube-ingress-aws-controller/pull/134 we could add the deletion protection in the same way to have some extra protection in place.

I would add this feature after #242 as it would apply to both ALB and NLB.

[mikkeloscar]

I tried this and it's a little bit annoying to handle because if you enable it then the stack deletion fails if a stack is not to be used anymore. So you need to first disable this and then delete the stack..