Rover: allow WATER_DEPTH mavlink message rate to be specified

... and reduce the default rate

this is currently unconditionally streamed at 50Hz, chewing up all available bandwidth on some telemetry radios.

Rover/GCS_Mavlink.cpp

@@ -589,6 +589,7 @@ static const ap_message STREAM_EXTRA3_msgs[] = {
     MSG_WIND,
 #if AP_RANGEFINDER_ENABLED
     MSG_RANGEFINDER,
+    MSG_WATER_DEPTH,
 #endif
     MSG_DISTANCE_SENSOR,
     MSG_SYSTEM_TIME,

Rover/Log.cpp

@@ -79,10 +79,6 @@ void Rover::Log_Write_Depth()
                             (double)(s->distance()),
                             temp_C);
     }
-#if AP_RANGEFINDER_ENABLED
-    // send water depth and temp to ground station
-    gcs().send_message(MSG_WATER_DEPTH);
-#endif
 }
 #endif