[ECS] Configure log driver for the amazon-ecs-agent container itself (awslogs, splunk, etc)

First of all, I really like the simplicity this project provides!

If I'm not mistaken, it is currently not possible to configure the logging of the agent container itself. The agent will pick up the ecs.config but I see no way to configure the agent's logging.

What I would like is to pass

```
--log-driver=awslogs \
--log-opt awslogs-region=${aws_region} \
--log-opt awslogs-group=${ecs_log_group_name} \
```

arguments to the agent container.

Did I miss something or is there a way to hook into the agent start?