Use case:
- Forcibly kill a running task that is a recipe build, which triggered a lot of children tasks for tests

Actual:
- The task itself is killed, but not the children tasks

Expected:
- Whole task tree is killed

AI:
- Add new flag in /cancel endpoint, to signal if the whole task tree should be killed.
  - When set to true, kill all the children tasks.
- Update Web UI and CLI to set it to true by default.

Challenge:
- As long as issue 808836 is not implemented, this will cause process leaks/unclean tear down. But we should be able to cope with this as long as the bots reboot the host after a task failure.