dockertest.dockercmd: More exception detail

Both NoFail and MustFail sub-classes provided very little
info. from exception in execute().  Added some overridable
static values with some simple default values.

Signed-off-by: Chris Evich <cevich@redhat.com>
subtests...cp: Fix style/comments.

Instead of naked try...except, rely on NoFailDockerCmd to
provide any debugging details.  Clarify comment regarding
command-line fed code-generation via inspect module.

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