Enable AWS NLB cross-zone load balancing

[dghubble]

Cross zone load balancing is a new feature available now that we use NLBs. According to the AWS blog announcement,

• Before: "Prior to this support, Network Load Balancer routed requests only to targets located in the same Availability Zone. This meant that in scenarios where the clients were caching the DNS information of a zonal Network Load Balancer, some targets could receive a disproportionally higher number of inbound requests."
• Enabled: "requests are distributed equally to your targets regardless of the Availability Zone in which they are located"

We're opting to enable cross zone load balancing for the apiserver and ingress.

Testing: Using it in prod 👍
Requires: terraform-aws-provider v0.11.0 or higher
rel: hashicorp/terraform-provider-aws#3537