Adds a new debconf chapter

This patch adds a new debconf chapter in the Debian docs.
It describe generalities about the Debconf OpenStack. It
explains how the following is handled:
- Database config (using dbconfig-common)
- RabbitMQ config
- API endpoint registrations
- keystone_authtoken config
- preseeding

Also, a number of screenshots are added.

Note that this is a break-down of:
https://review.openstack.org/54394/

into smaller patches.

backport: havana
Change-Id: Ia5d8e5fc3d397f9441ecb50ef4a3458672307bdc
(cherry picked from commit c9728d9539752e828f88f4bec428e84346d28f47)
Merge "Adds a new debconf chapter" into stable/havana