always use an inventory file

Using "127.0.0.1," as the argument to `ansible-playbook -i` uses an
undocumented feature of Ansible. Michael DeHaan has indicated that
support for it may be pulled in the future
(https://groups.google.com/forum/#!topic/ansible-project/G_9JRGp5jGE). This PR future-proofs ansible-local against such a change.