Update Ansible provisioner to set cwd to staging directory.

If you want to use a custom ansible.cfg, Ansible will look in the following:

* ANSIBLE_CONFIG (an environment variable)
* ansible.cfg (in the current directory)
* .ansible.cfg (in the home directory)
* /etc/ansible/ansible.cfg

Since the provisioner wasn't running in the staging directory, an uploaded ansible.cfg would not work.