dockertest.dockercmd: cmdresult property

Hide the underlying behavior of execute() by always
returning a cmdresult reference, instead of a new instance.
Maintain API compatibility by making the cmdresult attribute
into a property.  Most methods in AsyncDockerCmd now
update values in the referenced cmdresult.  However,
getting up-to-the-second stdout/stderr still requires either
a manual call to those properties or to update_result().
Most of this is to help the __str__ result always represent
the almost-current state of the underlying command (including
stdout/stderr).  Updated unittests to match.

Signed-off-by: Chris Evich <cevich@redhat.com>