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>
It still seems to be trying to guess/anticipate at needs and uses too much.  It would be much more general and accurate to wait until you have a CID.  Everything after that point is 100% dependant on what you're interacting through, which doesn't sem to be the subject of this class.  It would be as trying to determine a network connection is functional by waiting on an incoming ftp transfer to occur.   Using that anology, this ``is_responsive()`` should be checking if an ARP reply comes back from the remote side.  In other words, this class seems much more about the "transport" than the "mechanism" using it.