When using the Neutron WSGI module, the ML2/OVN maintenance worker needs
to be spawned in a separate service. This patch adds the service
``neutron-ovn-maintenance-worker``, that is a single process service tha
runs the ``MaintenanceWorker`` instance. This process is in charge of
performing periodic routines related to the ML2/OVN driver.
This new service should be included in any deployment project that
allows to spawn Neutron ML2/OVN with WSGI. Along with this patch, a new
one for devstack will be proposed.
Related-Bug: #1912359
Change-Id: Iea2995adb3343aae74a1b617fbccfce5c62c6b87