Add functions for creating node VMs

The functions introduced with this changeset create and configure
VirtualBox node VMs.

The VMs use copy-on-write disks based on previously created base disks.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: Idfb78f3683c5bedba96f16fdef23512be6c251d9
Merge "Add functions for creating node VMs"