Support OpsWorks app environment variables

This feature was added few months ago:
http://blogs.aws.amazon.com/application-management/post/Tx2G95J53SKI2E0/AWS-OpsWorks-supports-application-environment-variables

And you can find sample Chef usage in AWS official cookbook. e.g.  aws/opsworks-cookbooks#d10a34f9d2ae7f61961e82d03b415653568196ee