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>
Hmmmm, this is not the first time I have seen need for ``iterations`` in a sub-subtest.  We have one built-in for ``Subtest`` but it is rarely used.  I'll open an issue on this, please share you comments on how it should work (from SubSubtestCaller perspective)...