It's a bit confusing to have Terraform poll until instances come up on
ASG creation but not on update. This changes update to also poll if
min_size or desired_capacity are changed.
This changes the waiting behavior to wait for precisely the desired
number of instances instead of that number as a "minimum". I believe
this shouldn't have any undue side effects, and the behavior can still
be opted out of by setting `wait_for_capacity_timeout` to 0.
The `ForceDelete` parameter was getting sent to the upstream API call,
but only after we had already finished draining instances from
Terraform, so it was a moot point by then.
This fixes that by skipping the drain when force_delete is true, and it
also simplifies the field config a bit:
* set a default of false to simplify the logic
* remove `ForceNew` since there's no need to replace the resource to
flip this value
* pull a detail comment from code into the docs
Paul Hinze