Set DucktapeJSONEncoder as default in reports generator.

[ivandasch]

Currently, there is inconsistency with json encoder in tests/reporter.py. Some methods use it, some uses default.
I.e. JSONReporter use DucktapeJSONEncoder, but other encoders not.

Lets introduce some alias to json.dumps i.e.

def json_dumps(obj, **kwargs):
    """
    Dump object to json string with Ducktape json encoder.
    """
    assert 'cls' not in kwargs, "Passing custom json encoder is forbidden."

    return json.dumps(obj, cls=DucktapeJSONEncoder, **kwargs)

and use it everywhere in tests/reporter.py

[ivandasch]

@ewencp Hi, could you please review this small patch?