Add step result to structured log artefact

The ```RESULT_DICT```` should be imported from buildbot.py. The structure can be found here https://github.com/mozilla/treeherder-service/blob/master/treeherder/etl/buildbot.py#L3

If this is the only structure you need from buildbot.py import it explicitly:

```from treeherder.etl.buildbot import RESULT_DICT```