Code review comment for lp:~mbp/bzr/597791-http-tests

After some discussion with Robert and bug 658044, I think I can make this converge a bit more with what's already done in testscenarios:

 * name the attribute 'scenarios'
 * make it an iterable of scenarios directly, rather than a list of TestVariations to multiply
 * generate the cross product by calling a function rather than implicitly; to work well with scenario generators that come from registries this function should be quite lazy
 * change the variations to generators or functions rather than classes

Also stylistically
 * clearer tests
 * remove extra VWS
 * module docstring