Fixes #1000: Uploading the whole ansible playbook directory

The change is quite small but very useful. Ansible playbooks tend to have a strict directory structure with roles, variables etc. Most of the time you just want to upload the whole structure, not parts.

My configuration now looks like this:
```json
"provisioners": [
    {
      "type": "ansible-local",
      "extra_arguments": "--limit {{user `image_name`}}",
      "playbook_dir": "ansible",
      "playbook_file": "ansible/{{user `image_name`}}.yml",
      "inventory_file": "ansible/local_inventory.yml"
    }
  ]
```