ha-nat_subnets_cidrs - dont support list of 1-2 elements.

[dmatusiewicz]

Problem with:
https://github.com/cartest/terraform-buildtools

How to reproduce:

• Clone: https://github.com/cartest/terraform-test.git
• Checkout branch: dawidm/tests
• cd cartest/terraform-test/test-buildtools/issue1
• terraform get
• terraform apply
• Expected error:

Error applying plan:
2 error(s) occurred:
index 1 out of range for list module.ha-nat.subnet_ids (max 1) in:
$
{module.ha-nat.subnet_ids[count.index]}
* index 2 out of range for list module.ha-nat.subnet_ids (max 1) in:

${module.ha-nat.subnet_ids[count.index]}
Terraform does not automatically rollback in the face of errors.
Instead, your Terraform state file has been partially updated with
any resources that successfully completed. Please address the error
above and apply again to incrementally change your infrastructure.

How to apply fix:

• terraform destroy
• edit file main.tf in step1 directory "step1"

(...)

source = "git::https://github.com/cartest/terraform-buildtools?ref=dawidm/tests"
(...)

• rm -rf .terraform
• terraform get
• terraform apply

We should have to consider,address and document the limitation that we have in buildtools module.
we can create buildtools module only with fixed list of: ha-nat_subnets_cidrs

• if we define 3 element list we cant extend or shrink or extend the list and make apply. The module will throw an error.
• it is not a big deal so my sugestion is to document this problem in README.md