[ECS] [request]: Enable Fluentd by Default

**Tell us about your request**
Enable the Fluentd Docker Log Driver by default in ECS.

Enabled Log Drivers are set with the `ECS_AVAILABLE_LOGGING_DRIVERS` config key in `/etc/ecs/ecs.config`.

Right now, you have to add user data to your instance:

```
#!/bin/bash
echo "ECS_AVAILABLE_LOGGING_DRIVERS=[\"awslogs\",\"fluentd\"]" >> /etc/ecs/ecs.config
```

The Fluentd Log Driver has been "available" in Docker [for several years now](https://github.com/moby/moby/commit/361a582ba0bccea04a8ea1799e68779fa66abb9f#diff-2307762cca30a588bb99fb9cef2ff63b); AWS has put out tutorials recommending it be used with [Fluent Bit](https://aws.amazon.com/blogs/opensource/centralized-container-logging-fluent-bit/) or [Fluentd](https://aws.amazon.com/blogs/compute/building-a-scalable-log-solution-aggregator-with-aws-fargate-fluentd-and-amazon-kinesis-data-firehose/) for container logs.

**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?**
I'd like a simple usability improvement- enable the Fluentd (and others that have been around for a while) log drivers by default.

**Are you currently working around this issue?**
With user data:

```
#!/bin/bash
echo "ECS_AVAILABLE_LOGGING_DRIVERS=[\"awslogs\",\"fluentd\"]" >> /etc/ecs/ecs.config
```