[ovn_central,ovn_host] Declare the OVN units in the services tuple

[suraj-cmd]

Both plugins call add_journal() for their unit — ovn-northd and
ovn-controller respectively — but neither declares a services tuple, and
neither collects the service status. An sosreport from a host where the daemon
has failed to start therefore has the journal but nothing showing whether the
unit is loaded, enabled or running.

Plugin._collect_services() runs each entry of the tuple through
is_service() and calls both add_service_status() and add_journal(), so
declaring the unit adds the missing status and replaces the explicit call.

It also gives both plugins an enablement trigger beyond the package name, which
matters here because they are written for deployments where OVN runs in
containers: the package names differ between distributions and are frequently
absent on a containerised host, while the unit name is the same.

The tuples are declared on the shared base classes so they apply to both
distribution subclasses.

Companion to #4439, which made the same change for openvswitch. Both plugins
are grouped in one PR since they are the two halves of the same stack and the
change is identical.

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[packit-as-a-service]

Congratulations! One of the builds has completed. 🍾

You can install the built RPMs by following these steps:

• sudo dnf install -y 'dnf*-command(copr)'
• dnf copr enable packit/sosreport-sos-4453
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.