Align ceilometer charm with worker-multiplier usage as found
across the majority of OpenStack Charms.  Worker configuration
values are set for API, Notification and Collector services
as needed for each release. Note that for Ocata or later, API
services are executed as WSGI apps under Apache which was
already plugged into the standard worker calculation logic
in charm helpers.

This includes dropping of the existing api-worker configuration
option in preference to a calculated approach.

Change-Id: Idba7c3012d9d24b7ae6ef8eade4a5261ef1cbddc
Closes-Bug: #1745189