Handle ELB instance deregistration

We've noticed in our production environment that we have a need for something to deregister nodes from load balancers as part of the draining procedure, before the instance is terminated. We're currently using [lifecycle-manager](https://github.com/keikoproj/lifecycle-manager) for this, but it would be nice if this was handled by the AWS Node Termination Handler instead.

The reason this is needed is that if the instance is terminated before it's deregistered from an ELB, a number of connections will fail until the health check starts failing. This is particularly noticeable on ELBv2 (NLB+ALB), which seem to take several minutes to react, so we need to have fairly high timeout times on the health checks.

The behaviour we're looking for is that the node termination handler finds a list of classic ELBs and target groups that it's a member of, sends a deregistration request and then waits for the deregistration to finish before marking the instance as being ready to terminate.