undercloud-deploy: Use process-templates.py from t-h-t root
undercloud-deploy: Use process-templates.py from t-h-t root

The command was assuming that process-templates.py was available from
the directory where it's being ran. This might not be the case, so
instead we attempt to get it from the t-h-t directory.

Change-Id: Ib315ac8247b93537a2492ac618e93af31681a516

openstack/python-tripleoclient