The behaviors of creating and updating mapping cell0 differ

Description
===========
The behaviors of creating and updating cell0 differ. Given that the parameters to nova-manage did not change, I would have expected no changes made to the cell_mappings table other than updating updated_at.

Steps to reproduce
==================
NOTE: Be sure to replace <MUST_BE_A_VALID_NOVA_API_DB_CONNECTION_STRING> in api.conf with a valid db connection string.
OPTIONAL NOTE: In order to restart nova-api and run "nova service-list" successfully, replace "mysql+pymysql://nova:novapwd@vip/nova?ssl_ca=/etc/ssl/ca-bundle.pem" in api.conf with a valid db connection string.

controller1:~ # mkdir -p /etc/novabug
controller1:~ # cat << EOF > /etc/novabug/api.conf
[database]
backend = sqlalchemy
connection = mysql+pymysql://nova:novapwd@vip/nova?ssl_ca=/etc/ssl/ca-bundle.pem
[api_database]
connection = <MUST_BE_A_VALID_NOVA_API_DB_CONNECTION_STRING>
EOF
controller1:~ # cat << EOF > /etc/novabug/nova.conf
[DEFAULT]
transport_url = rabbit://rabbit:rabbitpwd@vip:5671/
EOF
controller1:~ # chown -R nova:nova /etc/novabug
controller1:~ # /usr/local/bin/nova-manage --config-file /etc/novabug/nova.conf --config-file /etc/novabug/api.conf cell_v2 map_cell0
controller1:~ # mysql -D nova_api -e "select * from cell_mappings"
+---------------------+---------------------+----+--------------------------------------+-------+------------------------------------------------------------+----------------------------------------------------------------------------------------------------------+
| created_at | updated_at | id | uuid | name | transport_url | database_connection |
+---------------------+---------------------+----+--------------------------------------+-------+------------------------------------------------------------+----------------------------------------------------------------------------------------------------------+
| 2018-09-26 09:14:21 | NULL | 1 | 00000000-0000-0000-0000-000000000000 | cell0 | none:/// | mysql+pymysql://nova:novapwd@vip/nova_cell0?ssl_ca=/etc/ssl/ca-bundle.pem |
+---------------------+---------------------+----+--------------------------------------+-------+------------------------------------------------------------+----------------------------------------------------------------------------------------------------------+

controller1:~ # /usr/local/bin/nova-manage --config-file /etc/novabug/nova.conf --config-file /etc/novabug/api.conf cell_v2 update_cell --cell_uuid 00000000-0000-0000-0000-000000000000
controller1:~ # mysql -D nova_api -e "select * from cell_mappings"
+---------------------+---------------------+----+--------------------------------------+-------+------------------------------------------------------------+----------------------------------------------------------------------------------------------------+
| created_at | updated_at | id | uuid | name | transport_url | database_connection |
+---------------------+---------------------+----+--------------------------------------+-------+------------------------------------------------------------+----------------------------------------------------------------------------------------------------+
| 2018-09-26 09:14:21 | 2018-09-26 09:15:01 | 1 | 00000000-0000-0000-0000-000000000000 | cell0 | rabbit://rabbit:rabbitpwd@vip:5671/ | mysql+pymysql://nova:novapwd@vip/nova?ssl_ca=/etc/ssl/ca-bundle.pem |
+---------------------+---------------------+----+--------------------------------------+-------+------------------------------------------------------------+----------------------------------------------------------------------------------------------------+

Expected result
===============
Given that the parameters to nova-manage did not change, I would have expected no changes made to the transport_url and database_connection values of cell0.

Actual result
=============
After running "cell_v2 update_cell --cell_uuid 00000000-0000-0000-0000-000000000000", the transport_url changed from the hardcoded "none:///" from CellV2Commands.map_cell0() to the actual transport URL from nova.conf and the database_connection value lost it's string "_cell0" previously added by the nested function CellV2Commands.map_cell0().cell0_default_connection(). These changes cause ripple effects like the duplication of all rows in the output of "nova service-list" [as well as in Horizon]. See below.

controller1:~ # service nova-api restart
controller1:~ # nova service-list
+----+------------------+----------------+----------+---------+-------+----------------------------+-----------------+
| Id | Binary | Host | Zone | Status | State | Updated_at | Disabled Reason |
+----+------------------+----------------+----------+---------+-------+----------------------------+-----------------+
| 1 | nova-conductor | controller1 | internal | enabled | up | 2018-09-26T09:19:13.000000 | - |
| 2 | nova-scheduler | controller1 | internal | enabled | up | 2018-09-26T09:19:05.000000 | - |
| 3 | nova-consoleauth | controller1 | internal | enabled | up | 2018-09-26T09:19:07.000000 | - |
| 6 | nova-compute | compute001 | nova | enabled | up | 2018-09-26T09:19:09.000000 | - |
| 1 | nova-conductor | controller1 | internal | enabled | up | 2018-09-26T09:19:13.000000 | - |
| 2 | nova-scheduler | controller1 | internal | enabled | up | 2018-09-26T09:19:05.000000 | - |
| 3 | nova-consoleauth | controller1 | internal | enabled | up | 2018-09-26T09:19:07.000000 | - |
| 6 | nova-compute | compute001 | nova | enabled | up | 2018-09-26T09:19:09.000000 | - |
+----+------------------+----------------+----------+---------+-------+----------------------------+-----------------+

Environment
===========
Version: Current master branch

Workaround
==========
We are having to avoid using the --config-file parameters altogether and duplicate the cell0 logic from nova/cmd/manage.py in our own scripts by:
1) For "cell_v2 map_cell0", specifying --database_connection containing "_cell0"
2) For "cell_v2 update_cell" of cell0, specifying --database_connection containing "_cell0" and --transport-url of "none:///"
3) For creating or updating cell1, specifying --database_connection not containing "_cell0" and the actual --transport-url