HPCC-9612 Roxie is ignoring slaveThreads setting

The refactoring of roxie topology in rc13 introduced a regression where the
roxieSlaveThreads setting would be ignored, and only one slave thread would
be allocated.

This could lead to lockups in cases where a slave thread tried to make remote
calls, such as sqfilt.ecl in the regression suite.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4587 from richardkchapman/roxie-slave-threads

HPCC-9612 Roxie is ignoring slaveThreads setting

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>