cms aws.py: enable ADDL_INSTALLED_APPS for xblocks

This was something that I got stuck on on last week's hackathon.  @jarv or @e0d or @feanil to review.

Add the hook to the aws config so that adding ADDL_INSTALLED_APPS to the
dict will get picked up by the app.  This is already present in
lms/envs/aws.py, just moving this into cms/envs/aws.py to match.

I believe this will be needed to use xblocks in production with CMS.

Also helpful for devstack.  This could be configured in devstack in
private.py, but I prefer editing the cms.env.json directly, as that's
required for other things, like theming.  I don't use a private.py
anymore.