Configuration file for forum container

Summary:
Discourse uses a YAML configuration file to configure the Docker
image and container.

This template allows to regenerate /var/discourse/containers/forum.yml

Starting point is the standalone configuration shipped with the
Discourse Docker image repository. It exposes the port 32080,
uses Mandrill and install some plugins.

Plugins:

• docker_manager
• Mathjax

This template relies on getcredentials to get Mandrill credentials.

Test Plan: launch image with the configuration file

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D72