Remove, replace or make NAT gateways optional

[alexjfisher]

They're charged by the hour with each partial hour being rounded up. If in the course of chapters 6 and 7 you spin up and tear down the infrastructure a number of times (which test-kitchen does), this soon adds up.
eg over 2 days when I was doing those chapters (spending about 3-4 hours each day), I racked up 20 hours of NAT gateway charges which comes to $1.16 in my region. Everything else was included in the free-tier.
If I'd continued to play with test-kitchen, (I skipped the bit on adding a custom resource), I think readers will soon hit significant charges.

In general, it looks like a lot of aws students aren't happy with the unexpectedly high cost of using NAT gateways
https://forums.aws.amazon.com/thread.jspa?threadID=234959