Configure DNS servers

[mattludwigs]

Some providers will provide DNS servers, so we don't have to ask for them.

• Add configuration field :dns_servers which is a list of DNS IP address
• If this field is configured
  • Don't have usepeerdns option in the pppd options
  • Setup the VintageNet.NameResolver to use the provided IP(s)
  • Note: Right now in the PPPDNotifications module we assume that we always ask
    for DNS servers from the peer, we will need to not make this assumption and check those values before trying to set up the VintageNet.NameResolver
• If the field is not configured everything should function as it does right now