clear screen rc file every time you run stack.sh

fixes bug 1032022.

Devstack will not update service's start up command if the command already
exists in screen rc files due to previous deployment. There is no way to
clear a service's start up command so this would be a problem if the service's
start up command changes between different deployment. Clear the content of
screen rc file every time you run stack.sh to deploy openstack can solve
this problem.

Change-Id: I18d0700bb2169bdb0dadfa7982168aef2eff8b22
diff --git a/stack.sh b/stack.sh
index 774c454..4904576 100755
--- a/stack.sh
+++ b/stack.sh
@@ -1022,6 +1022,11 @@
     SCREEN_HARDSTATUS='%{= .} %-Lw%{= .}%> %n%f %t*%{= .}%+Lw%< %-=%{g}(%{d}%H/%l%{g})'
 fi
 
+# Clear screen rc file
+SCREENRC=$TOP_DIR/$SCREEN_NAME-screenrc
+if [[ -e $SCREENRC ]]; then
+    echo -n > $SCREENRC
+fi
 # Create a new named screen to run processes in
 screen -d -m -S $SCREEN_NAME -t shell -s /bin/bash
 sleep 1