Fix Ansible-local provisioner configuration check for 'playbook_paths'

There's a bug in the ansible-local provisioner. The config section status:

* playbook_paths (array of strings) - An array of paths to playbook files on your local system. These will be uploaded to the remote machine under staging_directory/playbooks. By default, this is empty.

From: http://www.packer.io/docs/provisioners/ansible-local.html

Trying this out locally on:

```yaml
{
  ...
  "provisioners": [
    {
      "type": "ansible-local",
      "playbook_file": "../ansible/self-contained.yml",
      "playbook_paths": ["../ansible/"],
      "role_paths": ["../ansible/roles/"]
    }
  ],
  ...
}
```

gave me this:

```bash
$ packer build packer.json 
virtualbox output will be in this color.

1 error(s) occurred:

* playbook_paths: ../ansible must point to a file
```

This commit fixes this