dockertest.output: Log stdout/stderr when __nonzero__()

As the checker is whastly based on stdout/stderr it's essential to have
the output in case of failure.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests...kill: Update & improve kill test

*  Added support for custom skipped_signals
*  Change the stress_cmd. Decrease the sleep timeout
   of the stress cmd (speedup the testing)
*  Add possibility to use ``os.kill90`` instead of
   ``docker kill`` as a test command.
*  Adds possibility to run main docker command
   as docker attach process.
*  Add 4 new tests for the sig-proxy
   feature of the main process (run or attach).
*  Improve skip_signals configuration
*  Add new test run_sigproxy test to check
   run/attach --sig-proxy feature.
*  Many Pylint/style cleanups and config. simplification
*  Update tests to work with latest docker behavior
   *  with tty=on `docker kill` => 0
   *  with tty=off `docker kill` => 255
   *  bash `kill -9 $cont_pid` => 137
   (Skip the exit code check until the behavior stabilizes)

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
subtests...kill: Enable quiet mode on most commands

So many docker commands run, the default console logging
becomes difficult to look at.  Mute console output by default
though details are still logged into debugging.

Signed-off-by: Chris Evich <cevich@redhat.com>