Fix SuspendExcNodes and SuspendExcParts on slurmctld SIGHUP.

On a reconfig, the exc_node_bitmap is cleared but then it was
not built again since last_work_scan was declared as a local static
variable in _do_power_work(). The fix is to make it global within the
plugin and reinitialize it to 0 on _init_power_config().

Bug 3078.