demo: add script to clean when demo is not stopped in a right way

demo/clean_demo.sh

+#!/bin/bash
+
+git clean -xf
+
+for i in ${screen_patterns[@]}
+do
+  screen -wipe $i;
+done;
+
+ip link|grep NETNS|awk '{print $2;}'|sed 's/.$//'|sed -E 's/(.*)@.*/\1/g'|xargs -I if ip link del dev if;