Add $TOP_DIR to the path when calling worlddump script

The missing reference to $TOP_DIR cause stack.sh to fail when called
from outside devstack's directory

Change-Id: I7faec7720896e7dcfe60fa87fb417f22c8801eef

Author: maurorodrigues

stack.sh

@@ -639,9 +639,9 @@ function exit_trap {
     if [[ $r -ne 0 ]]; then
         echo "Error on exit"
         if [[ -z $LOGDIR ]]; then
-            ./tools/worlddump.py
+            $TOP_DIR/tools/worlddump.py
         else
-            ./tools/worlddump.py -d $LOGDIR
+            $TOP_DIR/tools/worlddump.py -d $LOGDIR
         fi
     fi