Fix reservation creation on DOWN nodes

Fix creation of advanced reservation of cores on nodes which are DOWN.
There seems to be a bug with reservations using a node list (e.g.
Nodes=something + CoreCnt=something).  The result is a reservation made that's
arguably broken; listing the reservation (scontrol show reservation) will show
"Nodes=" (blank) and "CoreCnt=0".

It's very easy to reproduce, just by doing the following against a node in a
DOWN (also tested with POWER_UP) state:
scontrol create ReservationName=tmp_res StartTime=now EndTime=now+600
Nodes=<some_non_idle_node> CoreCnt=1 Users=<some_valid_user>
scontrol show reservation.
bug 2078