subtests.docker_cli: Update and add couple of new kill tests

Hi guys,

This patchset modifies a bit the `DockerCmd` verbose behavior:
<pre>
original:
quiet: (nothing)
nonverbose: Executing docker run
verbose: Execute docker run -t -i fedora bash
         (program is running)
         Command: ...
         Executed: ...
         ...

new:
quiet: (nothing)
nonverbose: Execute docker run -t -i fedora bash
verbose: Execute docker run -t -i fedora bash
         (program is running)
         Command: ...
         Executed: ...
         ...

</pre>
The change is very small, the overall line length changes just slightly, but the information value is huge.

Then it modifies this test to use the verbose mode instead of (improperly used) quiet mode.

I also found couple of bugs either new or introduced by previous changes (Output()), they should be fixed now.

Last but not least I added couple of new tests, which are mainly variants of existing tests only with different angle. This is necessarily as the behavior is not the same and eg. `stress_parallel` works fine, but the same test with `os.kill` generates troubles (including docker daemon hangs, sometimes).

From my test runs these tests are failing:
 * 'run_sigproxy_stress_parallel' - https://bugzilla.redhat.com/show_bug.cgi?id=1122463
 * 'attach_sigproxy_stress' - https://bugzilla.redhat.com/show_bug.cgi?id=1122401
 * 'sigstop_sigproxy' - https://bugzilla.redhat.com/show_bug.cgi?id=1122401
 * 'sigstop' - https://bugzilla.redhat.com/show_bug.cgi?id=1122401
 * 'run_sigproxy_stress' - https://bugzilla.redhat.com/show_bug.cgi?id=1122401
 * 'bad' - https://bugzilla.redhat.com/show_bug.cgi?id=1122403
 * 'go_lang_bad_signals' - https://bugzilla.redhat.com/show_bug.cgi?id=1096276