[Neutron] Add a new Neutron service: neutron-periodic-workers

This new service is spawned when using Neutron WSGI module. This new
service executes the plugin workers inside a wrapper executor class
called ``AllServicesNeutronWorker``. The workers are executed as
threads inside the process.

Depends-On: https://review.opendev.org/c/openstack/neutron/+/922110
Related-Bug: #2069581

Change-Id: I6b76b7bcee1365c80f76231e0311406831f8ce41
1 file changed