Allow 'meta roles' or roles that dont do anything but depend on other roles

Allows roles to be nothing more than meta/main.yaml.

An example of why you might want to do this:

```yaml
# roles/py-deploy/meta/main.yaml
dependencies:
  - virtualenv
  - gunicorn
  - role: supervisor
    version: 3.0
```

Then in my app:

```yaml
# site.yaml
roles:
  - py-deploy
```

This pull request just adds the check for the meta directory as well.

-- Thanks,
Thomas Omans