Refactor conditional creation of NAT gateway for EKS

[vandrijevik]

In the Terraform module aws/nat_gateway, all resources used have a conditional count attribute, which is dependent on the variable uses_nat_gateway: if the variable is false, count = 0, and otherwise count is set to a value that creates resources.

This PR refactors the conditional creation to be specified on the entire module declaration itself (which is available since Terraform 0.13), in order to simplify the code inside the module itself. The end result is the same, although any users of the module might need to do state mv operations because the internal Terraform address of the resources changes (things that used to be module.eks-vpc-nat-gateway.aws_eip.nat-gw-eip[0] are now module.eks-vpc-nat-gateway[0].aws_eip.nat-gw-eip, for example).