Updates pre-download section
Updates pre-download section

This patch removes some of the packages that are currently
pre-downloaded to the basedisk even though they are not needed for the
training cluster.

Implements: blueprint openstack-training-labs
Change-Id: If3808436a218cc15ce2fd037ea52ec2fa5176a1e

harrisonfeng/training-guides,harrisonfeng/training-guides,Nabs007/training-guides,harrisonfeng/training-guides,Nabs007/training-guides,openstack/training-guides,Nabs007/training-guides,openstack/training-guides