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>
``output.wait_for_output(output_fn, pattern, timeout=60, timestep=0.2)`` It's repeatedly calls ``output_fn`` trying to match regex ``pattern`` against it's return value.  Similar to ``autotest.client.utils.wait_for()``, it checks in ``timestep`` intervals.  After a maximum ``timeout`` expires, it returns ``False``, otherwise on match it returns True.  So in effect, it's nearly identical to what this ``read_until...`` method does.