Fix nova cell message queue URL with separate notification queue

If using a separate message queue for nova notifications, i.e.
nova_cell_notify_transport_url is different from
nova_cell_rpc_transport_url, then Kolla Ansible will unnecessarily
update the cell. This should not cause any issues since the URL is taken
from nova.conf.

This change fixes the comparison to use the correct URL.

Change-Id: I5f0e30957bfd70295f2c22c86349ebbb4c1fb155
Closes-Bug: #1873255