Generate docs for projects' rootwrap.conf file

There's nothing introspectable in the projects or oslo.rootwrap, but all
projects that use rootwrap have a sample etc/($project/)?rootwrap.conf
file. This file is relatively simple to parse for allowed options and
comments.  It's not the best solution, but it's a solution.

Closes-Bug: #1253690
Change-Id: I51973a4b8f0ac6ea047c4ffc07647a10bdea92ee
Merge "Generate docs for projects' rootwrap.conf file"