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>
This mapping would be a great thing to stick onto the class as a static attribute.  It allows easy extension/adjustments and will help out with unittesting later.  I'd also suggest having a ``send_control_string()`` method might be handy as well, but it's not needed for this test, we can leave it out until later.