Update default cell_v2 cell when it already exists

Update default cell_v2 cell when it already exists

nova-manage cell_v2 create_cell just uses a dumb string comparison to detect
when a cell already exists. If there is a slight difference (e.g ordering of
params in the db uri query string) it can result in duplicate cells.

With this patch we should detect that the default cell already exists and
update it to use the current transport_url/database_connection instead of
attempting to create a new cell.

Change-Id: If6a32e87b19cb0edf683144367701a115657ad0a
Closes-bug: 1718912
(cherry picked from commit ebcaabcc5c5d2840128b3609c82c4a70a81ea0a0)