allow ignore changes for some fields like desired_capacit

[athom]

sometimes, in production, ECS instances will be auto scaling, and the desired_capacity already quite different.
it will be helpful to allow ignore changes like desired_capacity,
which makes the apply won't affect the current nodes number, but only other fields.
it is backward compatibility.

example usage

module "container_service_cluster" {
  source = "azavea/terraform-aws-ecs-cluster"
  ...
  ignore_changes = ["desired_capacity"]
}