Allow setting the number of heat engine workers

This will allow scaling stack operations up on a single host.

Change-Id: I8e9a3a7f86663fdda9b03c42e16b3e2189cbbfa5
Default engine workers to the number of CPUs

The engine is single-threaded and thus needs more workers to handle more
concurrent requests on a single machine.

Change-Id: I36117c3876f84a2ff7ba5576444fdbf7716cc69e