Document use of pre-built images
Document use of pre-built images

This can save time when running through the deployment process
multiple times, and will be required for supported product
deployments.

Change-Id: Ib1c20ed8a647f35f6b34942ed7dec290e203772c

bcrochet/instack-undercloud,dmsimard/instack-undercloud,dpkshetty/instack-undercloud,dmsimard/instack-undercloud,bcrochet/instack-undercloud,eprasad/instack-undercloud,eprasad/instack-undercloud,rdo-management/instack-undercloud,dpkshetty/instack-undercloud,rdo-management/instack-undercloud