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

Set progress to 0 in tasks that do not have progress which 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." (i.e. optional)

http://pubs.vmware.com/vcd-55/index.jsp?topic=%2Fcom.vmware.vcloud.api.reference.doc_55%2Fdoc%2Ftypes%2FTaskType.html