Revert "Default engine workers to the number of CPUs"

This reverts commit 56041687844c7c04bfcad974a07ac3ff35f99e5c.

Problems have been reported on systems using QPID. The gate only tests
RabbitMQ.

Change-Id: Iaf83dc7582835ce5bf4534116c918640da7373aa
Partial-Bug: #1321303
Merge "Revert "Default engine workers to the number of CPUs""