labs: osbash.sh options: drop -f, add -b

Add a -b option for building node VMs. This should make it harder for
users to accidentally overwrite their existing VMs.

It also allows us to drop the rather confusing "-f" option; "-w" without
"-b" is fast now, because no local building is ordered. To build node
VMs locally and Windows batch files at the same time, "-b -w" (or "-bw")
can be used.

Change-Id: I17ec78c0625f3657adaf670b7d58f2e4fde2248b