Proceed to login to the Spark Cloud and returns an access token.

def _login(self, username, password):
        """Proceed to login to the Spark Cloud and returns an access token."""
        data = {
            'username': username,
            'password': password,
            'grant_type': 'password'
        }
        r = self.spark_api.oauth.token.POST(auth=('spark', 'spark'), data=data, timeout=self.timeout)
        self._check_error(r)
        return r.json()['access_token']