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>
My bad, I did make this stuff work in DockerCmd, I was misremembering the trouble I had with it.  There are just two very minor things to fix here then:

*  Remove reference to ``_log``, I can't find it defined/used anywhere here(?)
*  Fix the ``prefix`` default to match docstring or fix docstring to match no-default parameter.

I'm with you on the shorter log lines, maybe improve that further by omitting the ``self.name`` part?  I'm not sure it really adds a lot more value, yet could increase message length quite a lot.  Also, it's really mostly of use just to test developers, who generally know how to use a debugger to find out the same.