neutron: Add neutron-ovn-agent support

ansible/group_vars/all.yml

ansible/inventory/all-in-one

ansible/inventory/multinode

ansible/roles/neutron/defaults/main.yml

ansible/roles/neutron/handlers/main.yml

ansible/roles/neutron/tasks/config.yml

ansible/roles/neutron/templates/neutron-ovn-agent.json.j2

ansible/roles/neutron/templates/ovn_agent.ini.j2

doc/source/reference/networking/neutron.rst

releasenotes/notes/neutron-ovn-agent-d66236e44bde2c29.yaml

tests/templates/globals-default.j2

tests/templates/inventory.j2