'--database_connection' changes when update cell with '--transport-url' only

[Eric Xie]

Description
===========
I updated existed cell with '--transport-url' only.
Then the 'database_connection' changed to value same as cell0.

Steps to reproduce
==================
* I created one cell
nova-manage cell_v2 create_cell --name cell2 --database_connection mysql+pymysql://nova:XXX@172.16.1.20/nova --transport-url rabbit://openstack:XXX@172.16.1.21:5672/
* I updated cell2
nova-manage cell_v2 update_cell --cell_uuid 53c5c34d-b3c2-496f-986f-166e1d4d3845 --transport-url rabbit://openstack:XXX@172.16.1.19:5672/
* Check cells
nova-manage cell_v2 list_cells

Expected result
===============
+-------+--------------------------------------+-------------------------------------------+--------------------------------------------------+
| Name | UUID | Transport URL | Database Connection |
+-------+--------------------------------------+-------------------------------------------+--------------------------------------------------+
| cell0 | 00000000-0000-0000-0000-000000000000 | none:/ | mysql+pymysql://nova:****@172.16.1.14/nova_cell0 |
| cell2 | 53c5c34d-b3c2-496f-986f-166e1d4d3845 | rabbit://openstack:****@172.16.1.19:5672/ | mysql+pymysql://nova:****@172.16.1.20/nova |
+-------+--------------------------------------+-------------------------------------------+--------------------------------------------------+

Actual result
=============
+-------+--------------------------------------+-------------------------------------------+--------------------------------------------------+
| Name | UUID | Transport URL | Database Connection |
+-------+--------------------------------------+-------------------------------------------+--------------------------------------------------+
| cell0 | 00000000-0000-0000-0000-000000000000 | none:/ | mysql+pymysql://nova:****@172.16.1.14/nova_cell0 |
| cell2 | 53c5c34d-b3c2-496f-986f-166e1d4d3845 | rabbit://openstack:****@172.16.1.19:5672/ | mysql+pymysql://nova:****@172.16.1.14/nova |
+-------+--------------------------------------+-------------------------------------------+--------------------------------------------------+

Environment
===========
# apt list --installed | grep nova
...
python-nova/unknown,unknown,now 2:17.0.7-6~u16.01 all [installed,automatic]

[Takashi Natsume (natsume-takashi)]

I tried it at master (commit 6e4ab9714cc0ca147f61997aa7b68f88185ade5c).

The following message was shown.

stack@devstack-master:/opt/devstack$ nova-manage cell_v2 update_cell --cell_uuid b97750a1-6fee-4a8b-a675-f4b3184ade71 --transport-url rabbit://openstack:XXX@172.16.1.19:5672/
The specified transport_url and/or database_connection combination already exists for another cell with uuid 00000000-0000-0000-0000-000000000000.

But the transport url should be updated in this case.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/662383