labs: comments for get_vb_version
labs: comments for get_vb_version

Add some comments to get_vb_version.

Change-Id: I049a82956674544a0096fcadf5e23120c076672f

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