Update NAT Instance AMI list
Update NAT Instance AMI list

- adds the following regions:
  ca-central-1
  ap-south-1
  eu-west-2
  eu-west-3

[finishes #162277773](https://www.pivotaltracker.com/story/show/162277773)

Co-authored-by: Christopher Hunter <0def5591f9c2d66a301475594170cde2183496e5@icloud.com>

pivotal-cf-experimental/bosh-bootloader,pivotal-cf-experimental/bosh-bootloader