Push route handling into overlay, a few more nits fixed (#581)

overlay/device.go

@@ -3,6 +3,8 @@ package overlay
 import (
 	"io"
 	"net"
+
+	"github.com/slackhq/nebula/iputil"
 )
 
 type Device interface {
@@ -11,5 +13,6 @@ type Device interface {
 	CidrNet() *net.IPNet
 	DeviceName() string
 	WriteRaw([]byte) error
+	RouteFor(iputil.VpnIp) iputil.VpnIp
 	NewMultiQueueReader() (io.ReadWriteCloser, error)
 }