CLC-2495 Make it possible to reset external apps for beta/test from CC production

https://jira.media.berkeley.edu/jira/browse/CLC-2889

This takes care of one missing piece of the puzzle. Currently we don't have a good URL to post to bCourses beta (or test) to reset their external apps to point to CalCentral dev (or QA). I've been hosting the XMLs for each server separately at my personal website. Weak.

If this goes in, we can at least use a team-accessible solution. The premise is that XML configs rarely change once they land on production. And bCourses beta/test can read XMLs from production, even though they can't reach dev/QA.

So Ops' update-the-bCourses-servers cron jobs can give bCourses beta-or-test URLs like:
https://calcentral.example.com/canvas/lti_roster_photos.xml?app_host=calcentral-dev.example.com
instead of:
http://some.dweebs.personal.com/dev-lti_roster_photos.xml