allow aws cloudformation package --use-json to affect artifacts uploaded to s3 as well

Due to issue #2934, we constantly run into yaml issues with `Mappings` that reference accounts that start with a zero. As a workaround to that yaml bug, we were going to just use `--use-json` instead. Unfortunately, when `aws cloudformation package` is uploading artifacts from referenced AWS::CloudFormation resources, it's hardcoded to use the yaml format, which triggers this leading zero bug. If you specify, `--use-json` when packaging a template, that setting should propagate to the cloudformation artifacts it uploads as well.

Specifically, the following line is what I believe needs to support json as well:

https://github.com/aws/aws-cli/blob/develop/awscli/customizations/cloudformation/artifact_exporter.py#L408