Fix public network setup

This fixes an issue where vagrant crashes when up-ing a VM in public network mode: if the network is already set up in dhcp mode, using netsh to set it up to dhcp will return 1, causing a failure in configure_networks.

This PR fixes this by checking whether dhcp is already set up. This also fixes a missing import (the WindowsError exception was undefined).

I am sure there is a better way to do this, but I am rather clueless when it comes to ruby.