Allow to customize cpuidset for VirtualBox VMs

[jochenseeber]

In VirtualBox machines there is a cpuidset setting that is set as follows:

config.vm.provider :virtualbox do |v|
    v.customize ["modifyvm", :id, "--cpuidset", "00000001", "000306a9", "00020800", "80000201", "178bfbff"]
end

This currently cannot be modified in Test Kitchen. The driver customizations should be extended so the following works:

driver:
    name: vagrant
    customize:
        cpuidset: ["00000001", "000306a9", "00020800", "80000201", "178bfbff"]

[cheeseplus]

@jochenseeber for something that specific we recommend using the vagrantfile_erb facility instead and providing a custom Vagrantfile.