Make autoscaler integration tests pass

These test started failing around the time !4488 (merged) was merged. The failure is in pulling the helper image, which is tagged with the review branch's head commit, and does not exist. We theorise that another test in the same block (x or 4) as these failing tests is loading the helper image into the local docker daemon, and with the tests introduced in the above MR, the test ordering/grouping was shuffled around, and the tests that load the helper image are no longer in the same block as these test.

FOR NOW, lets work around the failure by specifying a helper image that is known to exist in the registry.