Remove task/container overridden in DockerHostConfig

[richardpen]

In task/container overridden, it's copying the task/container struct
which has a lock, this could cause deadlock if some other go routine
acquired the lock right before copying and released the lock after the
copy. This commit moved the overridden part to ACS when unmarshal the
task from payload message.

Summary

Fix #1047

Implementation details

Removed the Overridden function, and added the same in TaskFromACS.

Testing

• Builds on Linux (make release)
• Builds on Windows (go build -out amazon-ecs-agent.exe ./agent)
• Unit tests on Linux (make test) pass
• Unit tests on Windows (go test -timeout=25s ./agent/...) pass
• Integration tests on Linux (make run-integ-tests) pass
• Integration tests on Windows (.\scripts\run-integ-tests.ps1) pass
• Functional tests on Linux (make run-functional-tests) pass
• Functional tests on Windows (.\scripts\run-functional-tests.ps1) pass

New tests cover the changes:
yes

Description for the changelog

Licensing

This contribution is under the terms of the Apache 2.0 License:

[samuelkarp]

Can you add a test case that covers reading a task with overrides from the state file and unmarshalling it correctly so that the overrides are properly applied?

[richardpen]

@samuelkarp Actually the overrides field of the container won't be set in the agent today. We don't even need the code here in this PR, we shall remove this part of code when the model is updated.