Migrate oslo.service away from Eventlet.

We voted to migrate oslo.service's Eventlet use case to futurist and cotyledon.

A specification has been written https://opendev.org/openstack/oslo-specs/src/branch/master/specs/epoxy/remove-eventlet-from-oslo-service.rst

The backend system mechanism was recently introduced into oslo.service, meaning that we are now ready to start the migration of the Eventlet calls.

https://review.opendev.org/c/openstack/oslo.service/+/937010

https://review.opendev.org/c/openstack/oslo.service/+/935783

The migration must also integrate functional tests and a documentation to ensure that everything works fine from an user experience point of view.