Support vagrant-vbguest plugin

Ensure the proxies are configured before [vagrant-vbguest](https://github.com/dotless-de/vagrant-vbguest) plugin. The plugin [hooks](https://github.com/dotless-de/vagrant-vbguest/blob/2159085b236bbde3cc819302ab3d356dc6b6171c/lib/vagrant-vbguest.rb#L49) to the `VagrantPlugins::ProviderVirtualBox::Action::Boot` action so it will get executed earlier than us now.

We need to hook to `VagrantVbguest::Middleware` if it is found. But as that action is only called on VM boot with VirtualBox provider, and the `Provision` action is still also called, we have to hook to both and add some flag for not configuring the machine twice.

Related issue: dotless-de/vagrant-vbguest#77