Update apt_pre-download script
Update apt_pre-download script

Refactor apt_pre-download script to classify the packages as per the
services and nodes for better debugging. This change makes it much
more convinent to go through the apt_pre-download.sh for adding,
debugging the issues.

Change-Id: I49f16953805a5895b813abfda2a33775c54236fc

openstack/training-labs,dguitarbite/OpenStack-Labs,openstack/training-labs,dguitarbite/OpenStack-Labs,dguitarbite/OpenStack-Labs,openstack/training-labs