providers/virtualbox: fix out of the box dhcp support

Most users trying DHCP with VirtualBox end up hitting #3083 and become sad.

Here we have two commits which should make them happy again!

Much more detail in the commit messages - happy to tweak and rework as needed - feedback welcome. :cake: 

Looking forward to getting that sucker closed! :dancers: 
`ProviderVirtualBox::Driver::Base#execute` normalizes newlines to for us - see https://github.com/mitchellh/vagrant/blob/1c884fa4e5a847549afa45350e37b0362c50feae/plugins/providers/virtualbox/driver/base.rb#L353-L355