add retry to sync_assets

Change-Id: I575e3a55111c99be981ace147038153606ff3c1e
Signed-off-by: michael zhang <zhangcheng@rbcon.com>
Reviewed-on: http://reviews.cloudfoundry.org/12979
Tested-by: CI Master <cf-ci@rbcon.com>
Reviewed-by: Pin Xie <pxie@vmware.com>