Acceptance volume cmp 967

Fixes https://clusterhq.atlassian.net/browse/FLOC-967
(the only other alternative I see is to go back to getting `Unit`s from `DockerClient` on the remote nodes, but not compare `Unit`s like for like, as I just don't see how we can construct a `Unit` with `Volume`s whose `node_path` can be known to be the same as what a remote node will have mounted on - so we could just compare the information we care about)