labs: make use of snapshot functionality

This changeset pushes some of the code from virtualbox.install_node
to the configuration files.

In doing so, the patch demonstrates how additional snapshots can be
configured just by editing config/scripts.*.

Implements: blueprint openstack-training-labs
Change-Id: I1f5f6d5aa3842da8fa94bd1a10a753bb80db023b
Merge "labs: make use of snapshot functionality"