Parametrize headless option for VirtualBox

If you need to start VirtualBox VMs without a GUI, for example, on a
remote computer, you can now set the 'headless' parameter in config.sh
to '1'.

DocImpact
Closes-Bug: #1304345

Change-Id: I868fa67335506a5e048d37411f5d5795b560a338
Merge "Parametrize headless option for VirtualBox"