Return napp metadata or None if not found.

def get_napp(self, username, name):
        """Return napp metadata or None if not found."""
        endpoint = os.path.join(self._config.get('napps', 'api'), 'napps',
                                username, name, '')
        res = self.make_request(endpoint)
        if res.status_code == 404:  # We need to know if NApp is not found
            return None
        if res.status_code != 200:
            msg = 'Error getting %s/%s from server: (%d) - %s'
            raise KytosException(msg % (username, name, res.status_code,
                                        res.reason))
        return json.loads(res.content)