flushing load image response before returning success

[yhlee-aws]

Summary

go-dockerclient's stream client used to handle the stream response before returning success for the API call. Imitating the same behavior for ImageLoad to make sure we don't run into the race condition between HTTP response reporting success and image load successfully completing.

Implementation details

flushing response.Body from ImageLoad call before returning success

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: N/A

Description for the changelog

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[yhlee-aws]

If close() mandates waiting for stream EOF, copy isn't necessary. However Closer interface does not dictate any such behavior, and this duplicates the previous behavior with go-dockerclient.