Deploy platform stack using existing S3 build artifacts

This PR modifies the platform template and cookbook to retrieve and deploy artifacts published to S3 by the existing build process. These changes are intended to be non-breaking with regard to the existing deploy process.

Meta-shell-code for a Jenkins job to deploy a platform cloudformation stack:
```shell
git clone chef-agent
cd chef-agent
export AWS_ACCESS_KEY_ID=foo
export AWS_SECRET_ACCESS_KEY=bar
export PLATFORM_ENV=test
bundle install
bundle exec knife cloudformation create platform-$PLATFORM_ENV-$BUILD_NUMBER --file templates/platform.rb -p Environment:$PLATFORM_ENV -p PlatformBuildNumber:$BUILD_NUMBER --no-parameter-prompts
```

Once this process is vetted as a replacement for the existing Jenkins deploy job(s), we can make the chef-agent repository private to facilitate further Chef development.