Fix tripelo-precise build script

The recent devstack-prereq refactoring made a wrong assumption
about the CWD of the script.  cd first.

Change-Id: Ib2210d00668df95c1d731b75f7dd6dc958729d4f

modules/openstack_project/files/nodepool/scripts/prepare_tripleo.sh

@@ -24,6 +24,7 @@ if [ -f /usr/bin/apt-get ]; then
     sudo apt-get update
 fi
 
+cd /opt/nodepool-scripts/
 ./install_devstack_dependencies.sh
 
 # toci scripts use both of these