Add VirtualBox runner option to disable cloning of base vm

I suggest adding an option to the virtualbox runner to disable the creation of a dedicated clone of the base vm, for example like so:

[runners.virtualbox]
  base_name = "my-virtualbox-image"
  base_snapshot = "my-image-snapshot"
  disable_snapshots = false
  disable_cloning = true

Scenario

In my case I am trying to set up a Windows 7 VM with NI LabVIEW to automatically build LabVIEW projects.

The NI LabVIEW Development System and the LabVIEW Application Builder, which are obviously needed to build a LabVIEW Project into a selfcontained Application, must be activated using a License Key and this activation is tied to the "computer" it was activated on.

If I clone the vm afterwards (or let the gitlab-runner clone the vm), the License manager says "activated for a different computer" and the build gets stuck because of LabVIEW displaying a evaluation screen and waiting for input (which you can't even see because it is run in a context without gui).

I would now like to disable cloning, so that the runner starts the original base vm, creates a snapshot to speed things up and then executes the build directly on the base vm.

Of course this would limit concurrent builds, but maybe this could be overcome by specifying an "array of base vm's", which the user would then have to create and test by himself.

I suspect more people will run into a similar situation if they try to use licensed software on their build VMs...