Include rootwrap.xml in the compute admin guide

There was documentation in the repo that describes how rootwrap works,
but it wasn't actually included in any of the OpenStack doc books as
far as I could tell. This patch includes it in the compute admin guide,
which is where the file is located.

Cherry-picked from https://review.openstack.org/20551

Change-Id: I5e5e0581719e965f0949494623ec4b1de1817348
Merge "Include rootwrap.xml in the compute admin guide" into stable/folsom