Rover: make the default period 8

thanks Tom!

APMrover2/Parameters.pde

@@ -522,5 +522,5 @@ static void load_parameters(void)
     ins.set_default_filter(5);
 
     // set a more reasonable default NAVL1_PERIOD for rovers
-    L1_controller.set_default_period(11);
+    L1_controller.set_default_period(8);
 }