[vcloud_director] fix progress reporting of tasks without progress

Set progress to 0 in tasks that do not have progress prevents the
rendering of the progress of these tasks failing with a nil error.

According to vcloud api "Read-only indicator of task progress as an
approximate percentage between 0 and 100. Not available for all tasks."

http://pubs.vmware.com/vcd-55/index.jsp?topic=%2Fcom.vmware.vcloud.api.reference.doc_55%2Fdoc%2Ftypes%2FTaskType.html
Merge pull request #2837 from alphagov/vcloud_director_fix_task_with_missing_progress_for_5-5

[vcloud_director] fix progress reporting of tasks without a progress value