Feature/cdodge/allow setting of session cookie name

Since this is a change in aws.py which is the configuration top-level file for production environments, we're not going to be able to test locally. @e0d @singingwolfboy can you code review and I'll see if I can deploy to stage to at least make sure it doesn't blow up. However we can't really test the bug fix in stage since the domain names there does not overlap, e.g. (stage-edge.edx.org and stage.edx.org), which is unfortunate.