[ECS]: Support distinctInstance placement constraint with blue/green deployment

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**Tell us about your request**
Currently there is no way to configure service deployed using blue/green CodeDeploy to keep single task per instance, which might be needed because performance of multiple tasks per instance is worse than of a single task.

**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?**
Trying to do a blue/green deployment for a service configured with distinctInstance constraint the scheduler reports

> service X was unable to place a task because no container instance met all of its requirements. The closest matching container-instance C encountered error "DistinctInstance placement constraint unsatisfied."

No source files, tests, or implementation entry points are identified in the request. Start by investigating how ECS blue/green CodeDeploy services apply the distinctInstance placement constraint, and define done as successful blue/green deployment while maintaining one task per container instance.

Written by the indexing model from the issue text.