fix bug 834300 - attachments from ATTACHMENT_HOST 

https://bugzilla.mozilla.org/show_bug.cgi?id=834300

Check the first commit for the logic diff without all the cleanup diff.

To spot check:
1. `vagrant provision` or `sudo puppet apply puppet/manifests/classes/dev-vagrant.pp`
2. Attach an image to a page
3. Go directly to its `mdn-local.mozillademos.org` url (e.g., https://mdn-local.mozillademos.org/files/1/HTML5_Logo_128.png) to confirm the ssl cert exception
4. Refresh the page with the image
5. Verify its `src` successfully comes from `mdn-local.mozillademos.org`
6. Edit the source of the page to remove the `mdn-local.mozillademos.org` portion of the `src` so it's a host-less URI
7. Save and refresh the page
8. The request to `developer-local.allizom.org/files...` should be redirected to `mdn-local.mozillademos.org/files...`
9. Eat cookies.