Dockerclient list volumes 289

Fixes #289 

In `flocker.node._deploy` the function `find_volume_changes` still needs to be updated where it currently performs comparison on volume names rather than the `AttachedVolume` object, but I suspect those changes should be done in a different branch, as the changes there will be quite substantial.
I think at this point the function can be shortened even further. I.e. you can use `converter.convert()` instead of building up the dict yourself.