Rover: send y accel in m/s/s as nav_pitch

this allows for easier tuning in MissionPlanner

APMrover2/GCS_Mavlink.pde

@@ -239,8 +239,8 @@ static void NOINLINE send_nav_controller_output(mavlink_channel_t chan)
 {
     mavlink_msg_nav_controller_output_send(
         chan,
-        lateral_acceleration,
-        0,
+        lateral_acceleration, // use nav_roll to hold demanded Y accel
+        ins.get_accel().y, // use nav_pitch to hold actual Y accel
         nav_controller->nav_bearing_cd() * 0.01f,
         nav_controller->target_bearing_cd() * 0.01f,
         wp_distance,