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>