Don't reuse tun interface

Need drop tun0 interface to restart the tunnel correctly
when inet interface change and vpn push dns or route all traffic

previous behavior:
  when rmnet0 is up, openvpn connect to x.x.x.x:pppp
  now we switch connection to wifi0, vpn connection can be broken
  then openvpn wait the keepalive value and try to reetablish connection
  but if we reuse the tun0 interface, the default route or dns can be on
  the tun0 interface and openvpn try to join x.x.x.x:pppp through himself

Change-Id: I3e714629a293cff296322a2feb681856876457d1