Mention that if no transport_url is provided then the one
in the configuration file will be used for command
``nova-manage cell_v2 simple_cell_setup [--transport-url <transport_url>]``,
just like that for other cell_v2 commands.
Change-Id: Ifededa59f7ffe5887e67e29b93f70fa70dfaef33
Chen
8992416f9d