Automatically download Jenkins and start it on random port

Got code from Jenkinsapi for doing that and adopted it for use by pytest.

Unfortunately, pytest requires passing fixtures to all test methods, so example is in one test for now:

```
def test_boolean_and_int_params(jenkins_server):
    with mock_api.api(__file__, jenkinsurl=jenkins_server) as api:
...
```

That requres adding parameter to all tests that use jenkins... :-1: