Comment 1 for bug 1794493

This is similar to https://bugs.launchpad.net/nova/+bug/1793423 but for cell0 and it had a review https://review.openstack.org/#/c/603998 submitted and it was abandoned because this behavior of update_cell taking options from the config file is designed on purpose. See the comments. It would be expected that the corresponding config file of that cell conductor (default nova.conf for cell0) would be used for the update of that cell. However since cell0 is a special case and it doesn't have a transport url, the update_cell command should check for this and not update the cell0's transport_url when using the default nova.conf (like in the default devstack setup), just like we don't allow disabling cell0.