replaces pyflakes with flake8 in tox, modifies various flocker/ source

replaces pyflakes with flake8 in tox, modifies various flocker/ source files for PEP8 compatibility

https://github.com/hybridlogic/flocker/issues/35

http://build.flocker.hybridcluster.net/boxes-flocker?branch=pyflakes-flake8-35
I'd wouldn't have thought it was necessary to specify the flake8 dependencies here. They should be automatically installed by `pip`.