[#66653770] Fix case where bootstrap vars is not specified.

Kind of ugly that the fix to an issue with the configuration syntax in vcloud-launch is finding its way down into Core, but this is the correct place to fix this in the immediate term.

Suggestions on how to fix this in the long term welcome. I'd suggest that the preamble script generation logic should be brought up into vcloud-launch, and we simply pass an already interpolated script into Vm#configure_guest_customization_section