Customization of image name that gets run

Right now, it looks like the image being run is a constant `AgentImageName = "amazon/amazon-ecs-agent:latest"`. It'd be useful if this could be changed with an environment variable or something so that it's simpler to test and use custom forks of the agent.

Start by locating `AgentImageName = "amazon/amazon-ecs-agent:latest"` in the ECS agent source and tracing where that image is used. Determine the supported override behavior and verify it with the relevant existing tests or launch path. Done means custom agent images can be selected without editing the source, while the current default still works.

Written by the indexing model from the issue text.