Replace for_each with count for producing ECS custom policies.

[nbeloglazov]

Current approach using for_each doesn't work if a policy contains reference to resource. In this case it throws the following error:

Error: Invalid for_each argument
│
│   on .terraform/modules/td/main.tf line 17, in resource "aws_iam_policy" "ecs_task_execution_role_custom_policy":
│   17:   for_each    = { for s in var.ecs_task_execution_role_custom_policies : index(var.ecs_task_execution_role_custom_policies, s) => s }
│     ├────────────────
│     │ var.ecs_task_execution_role_custom_policies is list of string with 1 element
│
│ The "for_each" map includes keys derived from resource attributes that cannot be determined until apply, and so Terraform cannot determine the full set of keys that will identify the instances of
│ this resource.

Looks like for_each in this case isn't really necessary as it's used only to convert a list to index => value map. And that's what count imitate as well.