tests(integration): document regression test for `clusters:update --auth=`

We already had a regression test to ensure the deis CLI wasn't just
sending the key path when updating the cluster. This change makes that
clearer and clarifies the output of the `CheckList()` test function.
Merge pull request #1525 from deis/test-regression-auth-update

tests(integration): document regression test for `clusters:update --auth=`