Rover: set a default filter frequency of 5Hz

this should help rovers on rough surfaces

APMrover2/system.pde

@@ -394,6 +394,11 @@ static void startup_INS_ground(bool force_accel_level)
 	ins.init(AP_InertialSensor::COLD_START, 
              ins_sample_rate, 
              flash_leds);
+
+    // set a lower default filter frequency for rovers, due to very
+    // high vibration levels on rough surfaces
+    ins.set_default_filter(5);
+
     if (force_accel_level) {
         // when MANUAL_LEVEL is set to 1 we don't do accelerometer
         // levelling on each boot, and instead rely on the user to do