Add vb_customize to extend customizations for virtualbox

In order to handle more complex virtualbox configurations have added
vb_customize to driver_config.  This allows you to add the following
example:
```
driver:
  vb_customize:
    - modifyvm, :id, --memory, 4048
    - modifyvm, :id, --nic2, intnet
```
To generate:
```
  Vagrant.configure("2") do |config|
    # ...

    config.vm.provider :virtualbox do |virtualbox|
      virtualbox.customize ["modifyvm", :id, "--memory", "4048"]
      virtualbox.customize ["modifyvm", :id, "--nic2", "intnet"]
    end
  end
```
You can also use other VBoxManage commands outside of modifyvm (e.g. createhd,
storageattach, etc).