[vcloud_director] Improve support for Tasks.

- add Organization#tasks
- add Task#cancel
- rename Task#expirity_time to expiry_time
- Task#progress is an integer
- add missing Task#start_time
- expose Task attributes introduced in 5.1 API
Merge pull request #2130 from nosborn/vcloud_director_tasks

[vcloud_director] Improve support for Tasks.