Add support for VMs without private/public networking

When VM does not have any explicit private or public networking defined in Vagrantfile, all services on any ports are obviously available on 127.0.0.1 by default, which is also IP used when SSH'ing to that VM.

Users might not want to setup any public/private IPs just because they want set up hostnames.

In general - is there any meaningful reason to not support this?