Add $TOP_DIR to the path when calling tools' scripts
Add $TOP_DIR to the path when calling tools' scripts

The missing reference to $TOP_DIR prevent us to run stack.sh from a different
directory.

Cherry-pick of dae115251828e035fcad229f9c9d78856fb867b7
Change-Id: Ie15b89734264789bef9a8efcfdbbf2cdb89c936e

williamthegrey/devstack