[FIXED JENKINS-24095] Ensure only numExecutors exist when reducing

When reducing the number of executors from M to N, previously it'd
kill all idle executors, and then create executors 0...N-1 unless
they already exist (busy executors).

This can lead to the situation where e.g. executor `#5` is busy and
retained, and when setting the number of executors to 3, 0...2 are
created, resulting temporarily having 4 executors.

This change prevents `#2` from being created in the example. When `#4`
is finished, it'll be killed anyway and a new executor `#2` created,
so after some time the executor indexes are cleaned up matching
expectations.