az "a" doesn't work sometimes

[beenryuu]

It seem "a" is used to describe which az subnet will be set up in.
https://github.com/asdaraujo/edge2ai-workshop/blob/master/setup/terraform/network.tf#L66

But I hit an Error told me some instance types are not supported in the "xxxx-a" az.
Can I add an optional Env var to describe postfix of az which user perfer to and use it for subnet setup?

If OK, I'll do the modification