Use conditional to set heat-engine workers
Use conditional to set heat-engine workers

This commit adds a conditional to check if heat-engine worker count is
set. This results in os_workers fact being used if it is unset. There is
similar logic in puppet/services/heat-api.yaml.

Change-Id: Ie9f1a9c54ab3e25dacecfd323b3f68b639265e8f

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates