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>
Let's leave the log formatting to the ``subtest`` (``SubBase`` really) class.  As you've seen this stuff is soooo contentious and hard to get right.  This is only called in 3-places, every one of them returns the value back to the caller anyway.  Callers are in a much better position to judge whether or not something should be logged.

FWIW, I tried something very similar recently in AsyncDockerCmd and after talking with Jiri, we decided it's not central to functioning of the class and will mostly just get in the way.  It's super-duper easy to add extra debugging/logging stuff in targeted spots later, no reason to rush it in.