Fix CloudTrail handling of custom policy URLs.

[danielgtaylor]

This updates the CloudTrail customizations to stop making HTTP requests
to fetch custom policies, instead relying on the updated behavior of the
CLI to automatically handle file and URL parameters for custom commands.

Before this fix, the CLI would fetch the data into a string, then the
CloudTrail customization would try to fetch the string as if it were
a URL, fail, and throw an error.

cc @jamesls @kyleknap

[kyleknap]

I like this change. Can you add a test that uses the BaseAWSCommandParamsTest to make sure that paramfile values like file:// notation now works for the cloudtrail commands? I think there is already one test class that uses BaseAWSCommandParamsTest: https://github.com/aws/aws-cli/blob/cloudtrail-policy/tests/unit/customizations/test_cloudtrail.py#L40

To create the custom policy in a local file, you can create a temp file with FileCreator in testutils.py

[coveralls]

Coverage increased (+0.02%) to 93.37% when pulling 47b285e on cloudtrail-policy into a22da2f on develop.

[jamesls]

I agree with Kyle here. Let's verify that the policy we pass into the botocore layer is the value we expect.

[danielgtaylor]

@jamesls @kyleknap please take another look. I've updated the test to assert the policy contents are from the file.

[coveralls]

Coverage increased (+0.05%) to 93.4% when pulling 34fd660 on cloudtrail-policy into a22da2f on develop.

[kyleknap]

Thanks! 🚢

[jamesls]