Add neutron-ovn-agent group to Kolla inventory

This fixes the following issue seen while pulling Neutron images, caused
by a kolla-ansible change [1] adding support for neutron-ovn-agent:

    'dict object' has no attribute 'neutron-ovn-agent'

CI: Make Rocky Linux jobs non-voting to allow this to merge.

[1] https://review.opendev.org/c/openstack/kolla-ansible/+/871297

Change-Id: I9e0a315fe2c9ff91bdeffb15adcc353a20204d9a