If you set up requests_mock to catch all requests (which I would
recommend) you sometimes get caught with things like file:// paths that
should be allowed to reach the filesystem.
To do this we should allow you to add a matcher that says a specific
route can bypass the catch all and do a real request.
This is a bit of a layer violation but I thought it was easy to start
with, then realized why it wasn't.
Change-Id: Ic2516f78413b88a489c8d6bd2bc39b8ebb5bf273
Closes-Bug: #1501665
b2026313e3