Search for a key in the key-value store.
        :param key: string
        :rtype: string

def search_kv_store(self, key):
        """Search for a key in the key-value store.
        :param key: string
        :rtype: string
        """
        data = {
            'operation': 'RETRIEVE',
            'key': key
        }
        return self.post_json(self.make_url("/useragent-kv"), data)['value']