subtests.docker_cli: Add --sig-proxy tests

Hi guys,

this pull request changes the old kill test to make it possible to run some of it's tests using `os.kill` on `docker run` and `docker attach`, thus test the `--sig-proxy=true` on already existing tests.

Additionally it adds basic test, which verifies `--sig-proxy` works how it should (booth, positive and negative tests).

The first commit modifies the `OutputGood` to log the command output in case of failure. This is really essential for log analysis.

Regards,
Lukáš
Documentation is missing for ``kill_name_prefix``, ``check_stdout``, ``docker_stress_cmd``, ``stress_cmd_timeout``, ``test_length``, ``run_container_attached``, ``attach_options_csv``.  Though there may be just a typo because I do see a line in docs for ``top_name_prefix``.

Also, same comment here as below.  Consider if all these *really* (practically) should be options at all.  It seems that many could be class-attributes instead, which would greatly simplify the configuration and documentation.  It's largely a subjective call, but I'd suggest asking yourself for each: "Is option **X** likely to need changing by a tester during the course of normal CI setup?".  If "no" or "maybe sometime", then just make it a class attribute because chances are, it'll never be touched apart from updating the test code itself.