Allow specifying AZs to use

[sync-by-unito]

This issue was originaly raised by a customer with whome I did an C* to Astra migration. We might get a patch from them on this one.

It is impossible to specify which AZs to use. We've run into the problem of needing 5 proxy instances, but the c5.xlarge default instance type is not available in us-east-1e, only in us-east-1f. Thus, the fifth instance creation results in the error:

Error: Error launching source instance: Unsupported: Your requested instance type (c5.xlarge) is not supported in your requested Availability Zone (us-east-1e). Please retry your request by not specifying an Availability Zone or choosing us-east-1a, us-east-1b, us-east-1c, us-east-1d, us-east-1f.

This is a bit of a nightmare if you need to scale up the instance count and suddenly find you have to change instance type, thus recreating all existing instances. Ideally this would be handled by doing a lookup on the aws_ec2_instance_type_offerings[1] Terraform datasource to ensure that the AZs you use in the networking submodule support the proxy_instance_type setting .

[1] https://registry.terraform.io/providers/hashicorp/aws/latest/docs/data-sources/ec2_instance_type_offerings

┆Issue is synchronized with this Jira Task by Unito
┆Components: Automation
┆Priority: Major