Correct job node_cnt value for job completion plugin

When using the jobcomp/script interface, we have noticed the NODECNT
environment variable is off-by-one when logging completed jobs in
the NODE_FAIL state (though the NODELIST is correct).

This appears to be because in many places in job_completion_logger()
is called after deallocate_nodes(), which appears to decrement
job->node_cnt for DOWN nodes.

If job_completion_logger() only called the job completion plugin,
then I would guess that it might be safe to move this call ahead
of deallocate_nodes(). However, it seems like job_completion_logger()
also does a bunch of accounting stuff (?), so perhaps that would
need to be split out first?

Also, there is the possibility that this is working as designed,
though if so a well placed comment in the code might be appreciated.
If the decreased nodecount is intended, though, should the DOWN
nodes also be removed from the job's NODELIST? - Mark Grondona