Fix TestOOMContainer on Windows

[fenxiong]

Summary

Fixing functional test TestOOMContainer on Windows: #1763.

Implementation details

The test was failing because docker failed to pull image "python:3-windowsservercore". Our test ran on windows server 2016 so it cannot pull an image built from windows server core 1709 (Version compatibility). Looks like recently "python:3-windowsservercore" points to the 1709 image version rather than 2016 version. This PR changes the test to use the 2016 one explicitly.

I tried the following on our test facility and ended up with the conclusion above:

docker pull python:3-windowsservercore -> fail (what we currently do)
docker pull python:3-windowsservercore-1709 -> fail
docker pull python:3-windowsservercore-ltsc2016 -> succeed

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:

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.

[fenxiong]

Linux unit test failure is caused by timeout and is not related to this PR. will go ahead and merge this.