make rejoin-stack.sh keep the same service tags

Currently rejoin-stack.sh can not keep the same screen service tags
as the first you deploy openstack due to that the stack-screenrc lack
proper command to configure screen's hardstatus. just delete the old
stack-screenrc so that function screen_rc can write proper initialize
command into stack-screenrc.

fix bug 1182597

Change-Id: I4cb4c6ded93a5c7b0bd39d65a754ddf86553463d
diff --git a/stack.sh b/stack.sh
index f2054d9..e4a7acb 100755
--- a/stack.sh
+++ b/stack.sh
@@ -836,7 +836,7 @@
 # Clear screen rc file
 SCREENRC=$TOP_DIR/$SCREEN_NAME-screenrc
 if [[ -e $SCREENRC ]]; then
-    echo -n > $SCREENRC
+    rm -f $SCREENRC
 fi
 
 # Initialize the directory for service status check