Sprinkling around some pyflakes

Common things that I noticed:
* The standalone `import cfme.web_ui.menu` should be replaced with `from cfme.web_ui.menu import nav`, `import ui_navigate as nav` should go away
* Imports need to be PEP8'd. flake8 doesn't catch this because it can't know what's builtin or not.
* `#!/usr/bin/env python2` should only be in scripts, and then it should be the canonical name of the python2 bin, `python`. 
  * `python` is always python 2. `python3` is always python 3.
  * `python2` is not guaranteed to exist if python 2 is installed.
*  Many unused imports removed