[OVN] Use the OVN agent in "neutron-tempest-plugin-ovn"

Now, only in master (Caracal 2024.1), the "neutron-tempest-plugin-ovn"
job used the OVN agent with the metadata extension, instead of the
OVN metadata agent. This functionality is implemented in the referred
link.

The OVN agent won't be used in stable releases.

Depends-On: https://review.opendev.org/c/openstack/neutron/+/898238

Related-Bug: #2017871
Change-Id: I92b85ec1ac609ada18fc1185e56f07c36d353518
8 files changed