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'm not opposed having it returning data packaged up inside another thing, because then it's really only returning one thing :)  How about re-using CmdResult() here?

Another idea:  if this method is tied to ``InteractiveBash``, you could more safely separate out Stderr/Stdout for use in CmdResult.  That would help support re-using of familiar patterns in tests that involving CmdResult instances.  No need to "re-invent the wheel" as it were.