pytest: don't write stdout/stderr to junit xml

failures are already printed during the tests, so we probably
don't have to capture them in the junit xml as well. Currently,
there's some noise in this file, such as the output from pulling
the image;

```
pulling alpine:3.9.3
Pulling from library/alpine - None
Pulling fs layer - None
Downloading - [>                                                  ]  27.86kB/2.757MB
Downloading - [==========>                                        ]  580.3kB/2.757MB
...
```

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pytest.ini

@@ -2,3 +2,4 @@
 addopts = --tb=short -rxs
 
 junit_suite_name = docker-py
+junit_logging = no