Add "trigger" information to a test_slot created from a Gitlab MR

Small change to enable a lookup of what triggered a certain test slot in the couchdb. The vision here is that an automatically run throughput test for a ci-test build can post a comment to the same discussion once it's done.
This however requires the throughput test handler to figure out where to post, thus I'm trying to make this information available via the couchdb.

Thanks to @rmatev for essentially telling me what to implement :)