Add Baymodel section to User Guide
Add Baymodel section to User Guide

Baymodel section

This section explains the parameters defined in the baymodel,
along with the values expected and the usage.

Partially implements: blueprint user-guide
Change-Id: I7cf1a2c9110b8ad777aac5e14d46c2323cbcc9f3

openstack/magnum,ArchiFleKs/magnum,openstack/magnum,ArchiFleKs/magnum