Add api_workers for each service to defaults

Render {{ openstack_service_workers }} for workers
of each openstack service is not enough. There are
several services which has to have more workers because
there are more requests sent to them.

This patch is just adding default value for workers for
each service and sets {{ openstack_service_workers }} as
default, so value can be overrided in hostvars per server.
Nothing changed for normal user.

Change-Id: Ifa5863f8ec865bbf8e39c9b2add42c92abe40616