fix sync_assets issue

Change-Id: Id97761134abf27506616b3f3bcd9e878abbb96c3
Signed-off-by: michael zhang <zhangcheng@rbcon.com>
Reviewed-on: http://reviews.cloudfoundry.org/13013
Tested-by: CI Master <cf-ci@rbcon.com>
Reviewed-by: Jing Gong <jgong@rbcon.com>