remove support for serial docker pull image

[sharanyad]

Summary

pre 1.9 docker versions supported only serial image pull. since we deprecated for docker versions < 1.9, we can remove support for serial pull and have concurrent image pull as default
see issue #1549 for details

Implementation details

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

NA

Licensing

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

[yhlee-aws]

Did we not have any integ/functional test coverage on serial pull?

[yhlee-aws]

Please check that all tests pass

[sharanyad]

Did we not have any integ/functional test coverage on serial pull?

We did have an integ test, which has been removed in this PR.
There are functional tests only for concurrent/parallel pull.