Layer Downloader now looks for AWS_CA_BUNDLE if it exists

[DevOpsChris]

Issue #, if available: #917

Description of changes: This enables the layer downloader to us AWS_CA_BUNDLE environment variable if it exists. It defaults to SSL verify True if it doesn't.

Checklist:

• Write Design Document - No. Doesn't meet reqs. (Do I need to write a design document?)
• Write unit tests
• Write/update functional tests
• Write/update integration tests
• make pr passes
• Write documentation

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

[DevOpsChris]

I notice that tox runs the make pr command. Is that sufficient, or do I run it alone on my dev box?

[jfuss]

This looks good over all. I just request a unit test that can verify we are reading the AWS_CA_BUNDLE env var properly and passing its value to the requests call. Mocking out os.environ to return a dictionary with this key and value is enough.

[DevOpsChris]

I plan on squashing once the testing is good.

[DevOpsChris]

Squashed & rebased on latest upstream/develop commit

[DevOpsChris]

I'm ready once the test passes

[jfuss]

Hey @DevOpsChris, I pushed a patch to your branch. The os.environ patch was not patching the environ correctly in the module. I spent some time trying to use the @patch.dict but I could not get it to work. So I ended up patching os in the module and mocking out the return value. I also added a tests that read the value and expected that request.get had the correct value for verify.

I will merge once the tests pass on Travis. Thanks for spending the time on this.

[DevOpsChris]

Hey thank you for finding that quirk out. It looks like Travis is temperamental, it failed on a Go language test. I recommend re-testing it, it seems to make it work again. I wonder if it's a virtual machine timeout issue.

[jfuss]

@DevOpsChris this happens from time to time. It's unclear why but playing whack-a-mole (rerunning) is our current way around it.