Restore ability to manually power down nodes, broken in 15.08.12
in commit b4904661

The patch introduced in commit b4904661 (not powering down dead node) has a bad side effect.  Adding the "(node_ptr->last_idle != 0)" condition prevents from powering down nodes with the following command:

scontrol update nodename=nX state=power_down

because the state update function relies on zeroing the "last_idle" variable when a power_down is requested (see src/slurmctld/node_mgr.c, line 1589).

Reverting this commit should solve the problem...but I let you decide...

Didier GAZEN