subtests.docker_cli.detach: New detach test

This patch adds basic detach test. There are 3 variants:
* basic - basic ctrl+p ctrl+q test
* stress - many iterations (currently only 3 as ctrl+p ctrl+q requires
           sleep)
* multi_session - uses 2 session attached to single container
                  simultaneously

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
`InteractiveBash` class just overrides the executed command. Usually (eg. iteractivity test) uses pure `InteractiveAsyncDockerCmd` and doesn't execute bash at all. So you need either put this into `InteractiveAsyncDockerCmd` or create another class which wraps `AsyncDockerCmd` and guess what, it's called `dexpect` and the concept is well tested for years as `pexpect`. And there are other derivates as `aexpect` and other simplified ones.

You asked for comparsion to do it the other way so I put the bare minimum into `InteractiveAsyncDockerCmd` and you don't like it either. So really don't know, what do you expect. For me personally the `dexpect` version is way to go, because you have `DockerCmd` which takes care of process control and `dexpect` session which is used for interaction with bash-like interfaces (it doens't need to be bash, sh, ... at all, it talks well with interactive python too). So right now it's more about the design. I don't like to have many methods in one class as it can get messy and sometimes you might be lost in whether this class is about the process or about the container (eg. `docker.kill()` will it send kill to the docker, or to the process I'm talking to on the other end of the session). @jzupka made a very good point in changing the behavior from creating new process to wrapping existing compatible process, I really like the idea and yes it works fine (yes, there are definitely some bugs and yes it will need to stabilize, but the amount of new code is way less then in this code. Because the most parts are just copied from `p/aexpect`.

PS: If you know about other method to extend `DockerCmd` than these 2, let me know. I'm open to new ideas (original `dexpect` looked way different than the present one as you can see because the commits are still there)