Enhancements to support tunnel/tap interface as physical ports

Agent modifications to
- support no-arp on physical/vhost interface. Agent reads the IFFLAGS
  from interface to find no-arp flag
  When interface is marked with no-arp, agent will send packets with
  pre-determined ethernet header
- raw-ip on physical interface. Allow agent to send/receive raw-ip
  packets without ethernet header on physical-port.
  This is used when physical-interface is of type TUN