[ECS] [request]: Configure CapacityProviderReservation when no instances are running and tasks are provisioning

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**
Would like to be able to configure the `CapacityProviderReservation` value that is used when no instances are running and tasks are provisioning.

**Which service(s) is this request for?**
ECS Cluster auto scaling

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
Currently `CapacityProviderReservation` will always be set to `200` when provisioning tasks when no instances are running in the auto scaling group. This results in at least 2 instances being launched. Ideally this could be set to `150` so that only 1 instance is launched. As is, if the 2nd instance isn't needed, it will just sit idle until scale in happens.

**Are you currently working around this issue?**
Just paying for the idle instance.

**Additional context**
#1220 ensures that an unneeded instance will run for at least 15 minutes.

Start by reviewing the requested CapacityProviderReservation behavior for ECS cluster auto scaling and the context in issue #1220. Confirm how provisioning tasks behave when no instances are running and define what configurable behavior and scaling outcome would count as done; this issue names no repository files or tests.

Written by the indexing model from the issue text.