labs: skip guestadditions for ssh install
labs: skip guestadditions for ssh install

When building an ssh basedisk, there is no need to find and attach the
VirtualBox guest-additions.

Change-Id: I7da729596e3aa6a3000fd10046ee986929b03084
Implements: blueprint openstack-training-labs

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