Extend TaskDesirer with Cancel (and perhaps Delete?)

[suhlig]

@julz @andrew-edgar This is probably not a complete story yet, but we wanted to capture the requirement that came up as part of the discussion in this PR.

TaskDesirer needs to able to cancel (and potentially delete) a task.

In Diego these are documented as DeleteTask and CancelTask.

AC:

• Deploy CF with Eirini
• cf push an app that has at least one task (see docs)
• Cancel the running task
• Check that the task is no longer running usingcf tasks my-app

/CC @JulzDiverse