RFC: Container class for simplification of container interaction v3

Hi guys,

this is 3rd version of the container interaction RFC started here: https://github.com/autotest/autotest-docker/pull/116

It's not yet intended for inclusion, it only shows the way I intended this to go. I had some difficulties to convince `DockerContainer` to be useful and not just a bunch of details, but by design it wasn't easy and the current solution is not very nice (I'd call it ugly).

I'd like to ask you to look at the last commit in particular. It shows how I'd tests to look like (I don't say exactly, but it looks simpler to me).

Kind regards,
Lukáš