provider/openstack: Use security_groups as native set when update

builtin/providers/openstack/resource_openstack_compute_instance_v2.go

@@ -601,9 +601,8 @@ func resourceComputeInstanceV2Update(d *schema.ResourceData, meta interface{}) e
 
 	if d.HasChange("security_groups") {
 		oldSGRaw, newSGRaw := d.GetChange("security_groups")
-		oldSGSlice, newSGSlice := oldSGRaw.([]interface{}), newSGRaw.([]interface{})
-		oldSGSet := schema.NewSet(func(v interface{}) int { return hashcode.String(v.(string)) }, oldSGSlice)
-		newSGSet := schema.NewSet(func(v interface{}) int { return hashcode.String(v.(string)) }, newSGSlice)
+		oldSGSet := oldSGRaw.(*schema.Set)
+		newSGSet := newSGRaw.(*schema.Set)
 		secgroupsToAdd := newSGSet.Difference(oldSGSet)
 		secgroupsToRemove := oldSGSet.Difference(newSGSet)