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>
I think this method is trying to do waaaay too much work.  It would make the test easier to read/maintain/debug if each of the steps were broken down into their own method.  For exmaple, ``initiate_ping(...)``, ``detach(...)``, ``attach()``, ``detach(...)``, ``attach(...)``, ``stop_ping(...)``, ...  Also, if any of those methods prooves useful to other sub-subtests later, they can easily be plucked up and dropped into the base-class w/o needing to change any other code.