Flocker app resource limits floc 961

Fixes FLOC-961
I think that strictly the `du` deferred should be returned so that the parent deferred doesn't fire until the assertion has completed.

I guess it works at the moment because `docker_client.list` blocks, but if it ever become async, then this test might not work.

I also think the `du` variable name is unclear. Maybe use `waiting_for_list` or `enumerating` instead.