Acceptance volume cmp 967

Fixes https://clusterhq.atlassian.net/browse/FLOC-967
In trying this my thoughts were clarified: I give `assert_expected_deployment` a dictionary, and it seems like it should either say "it's fine" or "these are the differences between that dictionary and the actual deployment". This exposes the implementation of the assertion method, and it is therefore a more important fix than I suggested above in my opinion.