Remove eventlet

After ceilometer have switch to oslo.messaging.

The remaining deps on eventlet are:

* alarm-notifier use event.spawn_n to run task in background
* all ceilometer service use oslo-incubator.service.Service that use eventlet.event to handle start/stop

The plan:
* replace the Service class by a ceilometer one that does the same without event.
* create a helper method to spawn method in background
* switch oslo.messaging executor by blocking