Supply node hostname to flocker-changestate

Ok. I think this is ready for review. Thanks @adamtheturtle for doing the flake8 fixes.

Fixes #283 
It's hard to say considering we don't have any other real `INode` implementations.  It seems like it will be annoying to have to visit each `FakeNode` and assign a hostname, though, so perhaps that's an argument against expanding `INode` now.