Internal Network for VirtualBox Provider

This pull request allows for a virtual box provider configuration that specifies a `private_net_type` of either `:intnet` or `:host_only`.  By default, it is :host_only, thus changing nothing in the nominal case.  However, this allows the user to specify that `:private_network`s be created as virtual box internal networks rather than host only networks.  It also allows v1 vagrantfiles to specify networks of :intnet type (to bring this same functionality to v1 vagrant files. 

This pull request borrows heavily from PR  https://github.com/mitchellh/vagrant/pull/1400. and https://github.com/mitchellh/vagrant/pull/1382.