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 never used multi-loop feature of autotest, because IMO it's less readable. So I don't require this feature either. Anyway usually you need some preparation (which could be done in initialize) and postprocess (which fits postprocess) so I guess the only requirement is to define the (x)range...