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
3e8db91a1e