added dhcp configuration option for private networks

We are using private (hostonly) networks to provide interconnectivity between multiple vms. As we do not want to care about managing ip-addesses, we were using this config option in vagrant <= 1.1:

```ruby
config.vm.network :hostonly, :dhcp
```

Unfortunately this feature was deleted, although all required functionality still exists in the provider code.

This is an attempt to bring this functionality back to vagrant.

Configuration for version 2 configs now as follows:

```ruby
config.vm.network :private_network, type: :dhcp
```