I added this because a vpn node will have at least two interfaces and we wouldn't want obfsproxy daemon to listen to both of them, according to the discussion here https://leap.se/code/issues/5714

When obfsproxy is deployed in vpn node, it happens that $dest_ip and $bind_address are the same. Essentially obfsproxy listens to an arbitrary port, then forwards traffic to the vpn port in the same interface.