wait for datapath device

so that when the router container restarts after a reboot it waits for
the proxy to create the datapath device.

Fixes #2113

prog/weaver/main.go

@@ -348,6 +348,8 @@ func createOverlay(datapathName string, ifaceName string, host string, port int,
 	case datapathName != "" && ifaceName != "":
 		Log.Fatal("At most one of --datapath and --iface must be specified.")
 	case datapathName != "":
+		_, err := weavenet.EnsureInterface(datapathName)
+		checkFatal(err)
 		fastdp, err := weave.NewFastDatapath(datapathName, port)
 		checkFatal(err)
 		bridge = fastdp.Bridge()