[vcloud_director] Add support for custom fields in vapp

This adds read/write custom properties support to vapp via ProductSections, as specified in http://pubs.vmware.com/vcloud-api-1-5/wwhelp/wwhimpl/js/html/wwhelp.htm#href=api_prog/GUID-E13A5613-8A41-46E3-889B-8E1EAF10ABBE.html