dockertest.dockercmd: cmdresult property

Hide the underlying behavior of execute() by trying to
returning a fresh cmdresult value when possible, instead
of possibly a reference.  Do the same thing for __str__
so that the most current, detailed info is always available.

For the AsyncDockerCmd class, make the cmdresult attribute
into a property that always contains current stdout/stderr and
exit_status.  Deprecated update_result() method since it's now
covered by cmdresult property.  Updated unittests to match changes.

Add new sub/sub-subtest methods ``log_xn()`` to facilitate printing
multi-line messages where look-n-feel are important.  i.e. allow
debug output to go out w/o any further mangling.

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