Old NAT AMIs for us-east-1 don't work with t2.micro

[adimarco]

The natAmiId for us-east-1 is ami-4f9fee26, which is a PV AMI that doesn't work with the default NAT instance type of t2.micro

I'm guessing the factory_default ami_cache.json just hasn't been rebuit in a while. I'll put in a PR with an updated ami_cache in a bit.

[sesas]

Thank you!

[sesas]

There should be a script to generate it. @gimballock should know more about that part.