Add --max-capacity support to migrate.sh

[BCTBB]

• Adds support for CAPACITY_MAX="${MAX_CAPACITY}", if scaling up. CAPACITY_MAX = 0, if scaling down.
• Adds support for --max-capacity in the ${ADJUST_AUTOSCALING} logic

[rfairburn]

If we are adding a new max capacity, then it needs to be passed in as a new variable to the module:

module "migrations" {
  source                   = "github.com/fleetdm/fleet-terraform/addons/migrations?depth=1&ref=tf-mod-addon-migrations-v2.1.0"
  ecs_cluster              = module.fleet.byo-vpc.byo-db.byo-ecs.service.cluster
  task_definition          = module.fleet.byo-vpc.byo-db.byo-ecs.task_definition.family
  task_definition_revision = module.fleet.byo-vpc.byo-db.byo-ecs.task_definition.revision
  subnets                  = module.fleet.byo-vpc.byo-db.byo-ecs.service.network_configuration[0].subnets
  security_groups          = module.fleet.byo-vpc.byo-db.byo-ecs.service.network_configuration[0].security_groups
  ecs_service              = module.fleet.byo-vpc.byo-db.byo-ecs.service.name
  desired_count            = module.fleet.byo-vpc.byo-db.byo-ecs.appautoscaling_target.min_capacity
  min_capacity             = module.fleet.byo-vpc.byo-db.byo-ecs.appautoscaling_target.min_capacity
  max_capacity            = module.fleet.byo-vpc.byo-db.byo-ecs.appautoscaling_target.max_capcity
  depends_on = [
    module.fleet, 
  ] 
} 

Then we roll that into the script from terraform and work with it. Otherwise, when we scale back up, we don't have our original max capacity from terraform.

If I am missing something here, please let me know, but I think the pr as is will cap the upper-end capacity lower than we want after scale-up.

[BCTBB]

I was leveraging ${COUNT} which looks tied to the ${DESIRED_COUNT} to drive the behavior for the --max-capacity, so that expected behavior is driven by the correctly labeled input.