Update devstack to use workers
Update devstack to use workers

Use API_WORKERS for all services, to make sure we
are testing with at least 2 workers for every service.

* Use API_WORKERS for all services.
* Removed deprecated and removed settings for the worker.
* Use enable_host_header is tests.

Change-Id: Iba9ba655c47ee085a47ee98872437f07fdc3346c

openstack/designate,openstack/designate,openstack/designate