Fix the nova.conf mustache template

The nova.conf mustache template is not correct in the section that enables the
passthrough settings.
The template has two problems:
- comments are not rendered
- as all variables are HTML-escaped by default in a mustache template we want
  to use triple braces to return unescaped HTML.

This matches https://review.openstack.org/#/c/91625/3
Closes-bug: #1324598

Change-Id: Ib5831d1fd26e0f5b2d023d7f892420b776892ca1
Merge "Fix the nova.conf mustache template"