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>
for me this loop is easier to read then 2 wait_for() functions (as we need internal and full timeout). Also the handling of internal timeout is pretty specific and there are lots of tests from virttest which can be reused in docker, when we have similar read_nonblocking behavior. (The behavior actually comes from the bash interactivity and it tries to fit it's output as much as possible to avoid unnecessary hangs).