Add Image Management section in User Guide
Add Image Management section in User Guide

Image as part of a driver for COE+distro.

Describe image as part of the driver for a particular COE
and particular distro:

-Interaction with templates, scripts.
-List software content and version.
-Point to guide for building images.
-How software are run: as service or container
-Where to download supported images.

Partially implements: blueprint user-guide
Change-Id: I39b912f7474e3b4e93c775f766dbd519da9656ec

jay-lau/magnum,ArchiFleKs/magnum,openstack/magnum,ArchiFleKs/magnum,openstack/magnum