Allow proper instance cleanup if state == SHUTOFF

Removes an obsolete check for instance's power_state
on shutdown_instance().  With it in place, volume detachment
and disassociation never takes place.  Compute should instead rely
on virt drivers to handle this case and raise accordingly. libvirt's
destroy() currently handles powered off instances fine, and properly
detaches any existing volume connections.

Fixes bug 954692

Change-Id: I200d5b2073e5b52a9733d8324d016b14bdc96067

nova/compute/manager.py

@@ -673,13 +673,6 @@ def _shutdown_instance(self, context, instance, action_str):
         # tear down allocated network structure
         self._deallocate_network(context, instance)
 
-        current_power_state = self._get_power_state(context, instance)
-
-        if current_power_state == power_state.SHUTOFF:
-            self.db.instance_destroy(context, instance_id)
-            _msg = _('trying to destroy already destroyed instance: %s')
-            raise exception.Invalid(_msg % instance_uuid)
-
         # NOTE(vish) get bdms before destroying the instance
         bdms = self._get_instance_volume_bdms(context, instance_id)
         block_device_info = self._get_instance_volume_block_device_info(