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.
:question: Does yaml have `None`? The user is not giving a Python dictionary so this is exposing the internals of the system.

I also think that quotes should be around the error-causing mountpoint - without it the error could look weird if the given string has whitespace at the beginning or end, or full stops for example.