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>
Unless...you're not docker running bash, then they're almost useless as part of ``InteractiveAsyncDockerCmd``.  For example, what if I wanted to ``docker run python`` or ``docker run vi`` interactively?  This is a great example of where more closely following the Abstract-base-class pattern could help.  It's strength is in supporting extension and customization across a wide variety of scenarios.  Otherwise, by keeping so much specific functionality bound up at a high-level,  other developer's hands are tied to only what you have imagined ahead of time.  It's much better IMHO to design-in expansion from the start, rather than limited usefulness.