Allow NATs to have EIPs

[sesas]

There's a use case for which EIPs attached to the NAT instances would be helpful: if an external service needs to include the IP of a trusted host in a whitelist, then the IP needs to be stable.

We could probably add a config flag to the nat section of the config, like assign_eip here: https://github.com/DualSpark/cloudformation-environmentbase/blob/master/src/environmentbase/patterns/base_network.py#L39