Stop duplicating Nova cells

Check if a base Nova cell already exists before calling `nova-manage
cell_v2 create_cell`, which would otherwise create a duplicate cell when
the transport URL or database connection change.

If a base cell already exists but the connection values have changed, we
now call `nova-manage cell_v2 update_cell` instead. This is only
possible if a duplicate cell has not yet been created. If one already
exists, we print a warning inviting the operator to perform a manual
cleanup. We don't use a hard fail to avoid an abrupt change of behavior
if this is backported to stable branches.

Change-Id: I7841ce0cff08e315fd7761d84e1e681b1a00d43e
Closes-Bug: #1734872