[ECS] [request]: ECS Service replacement update should retrieve desiredCount from current Service

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**Tell us about your request**
The DesiredCount property should be retrieved from current ECS Service when replacement update without desiredCount specified.

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

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
[issue-493](https://github.com/aws/containers-roadmap/issues/493#issuecomment-721421554)
We found the optional desiredCount already released one month ago. We already tested two scenarios in above link which works well.
However, here is another situation:
1. An ECS service is running with desiredCount equals to 5.
2. Replacement update was triggered when we apply new CFN without desiredCount specified.
3. New ECS service was created, but the desiredCount go back to 1 automatically.
Ideally, the new ECS service should keep the same desiredCount as previous service after update.

**Are you currently working around this issue?**
In our case, the desiredCount is mandatory which can not be ignored. Before update, we will retrieve desiredCount from current ECS service. If we want to leverage optional desiredCount, we need to set a value for it to keep the size all the time. Which is not optional actually.

Start by reading the linked issue-493 discussion and the two replacement-update scenarios described here. Reproduce an ECS Service replacement with desiredCount omitted from the new CloudFormation configuration, comparing the existing and replacement services. Done means the replacement preserves the current service's desiredCount without requiring the property to be specified.

Written by the indexing model from the issue text.