Fix stack name php deploy

Noticed that when deploying our PHP app to opsworks, the stack_name property of opsworks.php, in the shared deploy directory of our repo on our instances, was not being populated. Looked around, and found that in the php/configure recipe, the stack name is successfully passed into the template. Figured I'd go ahead and just copy the line over.

Let me know if I'm way off course. Thanks!
Dan