fix upload assets

1. do not need update assets before build
2. use multipart post to upload assets

Change-Id: I076f1416405a5a8d5f7e80b61ebb8662ce760627
Signed-off-by: Pin Xie <pxie@vmware.com>
Reviewed-on: http://reviews.cloudfoundry.org/13090
Tested-by: CI Master <cf-ci@rbcon.com>
Reviewed-by: Michael Zhang <zhangcheng@rbcon.com>