Gazebo client: Reduce priority to avoid affecting other processes

Tools/sitl_run.sh

@@ -107,7 +107,7 @@ then
 			# gzserver needs to be running to avoid a race. Since the launch
 			# is putting it into the background we need to avoid it by backing off
 			sleep 3
-			gzclient --verbose &
+			nice -n 20 gzclient --verbose &
 			GUI_PID=`echo $!`
 		fi
 	else