Restore ability to manually power down nodes

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