labs: rename local vars: virtualbox.install_node
labs: rename local vars: virtualbox.install_node

Make local variables lowercase and some of them more descriptive.

Change-Id: I14072c03243e0fc16aef4a5b6513ca530a99d911

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