change custom domain acm creation condition

[hong-yi]

The current condition when starting from an empty environment will always throw an error:

Error: Invalid count argument
on .terraform/modules/apigw.apigw_custom_domain/modules/custom_domain/main.tf line 39, in resource "aws_acm_certificate" "cert":
  count = var.create_acm_cert && var.cert_arn == "" ? 1 : 0
The "count" value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created. To work around this, use the -target argument to first apply only the resources that the count depends on

To fix this, fix the condition to only check for var.create_acm_cert and set it to true if required