code: def start_instance(self, key_name, public_key_path, private_key_path,
                         security_group, flavor, image_id, image_userdata,
                         username=None, node_name=None):
          
          pass
docs: Starts a new instance on the cloud using the given properties.
          Multiple instances might be started in different threads at the same
          time. The implementation should handle any problems regarding this
          itself.
  
          :param str key_name: name of the ssh key to connect
          :param str public_key_path: path to ssh public key
          :param str private_key_path: path to ssh private key
          :param str security_group: firewall rule definition to apply on the
                                     instance
          :param str flavor: machine type to use for the instance
          :param str image_name: image type (os) to use for the instance
          :param str image_userdata: command to execute after startup
          :param str username: username for the given ssh key, default None
  
          :return: str - instance id of the started instance