Enable support for custom log drivers

### Summary

Hello, our company needs to use a custom log driver designed to meet our requirements, but unfortunately, the ECS agent does not support custom log drivers.

### Description

Because Docker's built-in gelf log driver does not meet all of our company's requirements, we have developed a plugin for Docker, a custom log driver that transforms and handles the logs of applications running on Docker. One of the transformations is the aggregation of n partial logs into a single log containing the entire message. We also have resolved a known TTL DNS issue that affects the log driver gelf (Reported at issue https://github.com/moby/moby/issues/17904#issuecomment-194403940). Our solution is working, but when we tried to use the log driver on the ECS agent, we figure out that it does not support custom log drivers.

Maybe something should be done in this session code:

https://github.com/aws/amazon-ecs-agent/blob/f95f731bce74e6276f9fd8bfbb87a62d3a790ce8/agent/config/config.go#L295-L304

Start with the ECS agent session code in agent/config/config.go around lines 295-304 and review how Docker log drivers are configured. Compare that path with the requested custom Docker log driver support; done means the ECS agent can use custom log drivers without breaking existing logging behavior.

Written by the indexing model from the issue text.