[ECS] [AWS CloudMap [request]: Use an existing hosted zone for the CloudMap namespace when using service discovery

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**Tell us about your request**
When creating a namespace, CloudMap automatically creates a new private/public hosted zone for the records. This feature request is to allow the use of an existing hosted zone for the CloudMap (i.e. service discovery) records.

**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?**
There is an existing hosted zone used for inter-service communication using a bunch of public load balancers, and application knows about these load-balancer entries via the route53 zone. With current behaviour of creating a separate zone, the application would need to be updated to look at 2 zones.

It would also be useful to have records use an existing zone where a common domain would be used for services outside of CloudMap and ECS