Fix designate-producers not running properly

Kolla is setting the producer tasks to None, and this
disables all producer tasks.

Starting with the next release of Designate a new task
was added to the producer named increment_serial that
is now critical path for incrementing the serial for each zone.
https://github.com/openstack/designate/commit/fb9c2da977a36afc695b1e75a81aa85d8e56ef0a

Closes-Bug: #1879557
Change-Id: Ief8dca4e27197c9576e215cbd960da75f6fdc20c

ansible/roles/designate/templates/designate.conf.j2

@@ -49,7 +49,6 @@ workers = {{ designate_worker_workers }}
 [service:producer]
 workers = {{ designate_producer_workers }}
 threads = 1000
-enabled_tasks = None
 
 [network_api:neutron]
 ca_certificates_file = "{{ openstack_cacert }}"