[ECS, Fargate] Support awslogs-endpoint task configuration option

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

Fargate, ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

After @clareliguori helpfully pointed out that my issue #48 was mistaken and the issue was with the `awslogs` driver, I looked into the CloudWatch Logs VPCE support. Currently the only way to make it work is to update the VPC's DNS to point at the PrivateLink endpoint, which while fine for some situations, can be a bit coarse-grained for others (especially if there are nontrivial policies on the endpoint). As of a recent Docker ([this commit](https://github.com/moby/moby/commit/998478d369ad3784c3cd28038d836c3c94b16936) adds it), the `awslogs` driver now supports the `awslogs-endpoint` configuration option, which would allow me to point my task directly at the logs VPCE that's appropriate for it. If I try to pass that into my task definition today, it tells me immediately that the option is invalid.

**Are you currently working around this issue?**

Just using DNS for the VPCE.

Edit: a broader way to phrase this might have been "Support Docker 18.09", as long as the front-end validation for the task definition schema notices that the new option is present 😄