tests: First pass on making unit tests work again. - Fix all imports - Add more helper functions to work with mock file-systems - Simplify some code by running chef directly on the mock FS - Fix a couple tests

cov-core==1.15.0
coverage==4.0.3
invoke==0.12.2
mock==1.0.1
pytest==2.8.7
pytest-cov==2.2.1
pytest-mock==0.10.1