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: Message grooming

*  DockerCmd subclasses can now accept any SubBase
   derrived class.  They make use of this in logging
   messages with a more precise sub/sub-subtest name.
   Changed all calls to pass in the sub-subtest instead
   of always the parent subtest.

*  There's no need to print out all details for every single
   docker command when hundreds are run.  Defined local
   quiet/non-verbose versions of DockerCmd classes.  Enough details
   are still recorded into debug log to aid in finding any problems.

*  There were multiple nearly-identical messages printed
   by several sub-subtests.  The differences were non-
   substantial, and full details are always printed
   to the console.  Consolidated the "Missing signal" message
   and exception raising into a single method.  Removed
   details from exception and added to debug log.

*  Updated kill/stress to use Output class.

Signed-off-by: Chris Evich <cevich@redhat.com>
Merge pull request #201 from cevich/sigproxy

Close 65 and 167