fix of comp nodes causing backfill to end early

Completing nodes is removed when calling _try_sched() for a job, which
is the case in select_nodes(). If _try_sched() thinks the job can run
now but select_nodes() returns ESLURM_NODES_BUSY, the backfill loop will
be ended.
Merge branch 'slurm-2.6' into slurm-14.03