Chatops rollback check command should compare the last 2 successful deployments

@rspeicher I decided not to block gitlab-com/chatops!180 (merged) with minor remarks.

I've applied one suggestion myself and opened this follow-up to discuss a couple of improvements/comments.

The following discussions from gitlab-com/chatops!180 (merged) should be addressed:

• @nolith started a discussion:

  It seems that we do not support ordering by finished_at :(

• @nolith started a discussion:

  I will suggest filtering by status, we mentioned quite a few times to start triggering the start of a deployment. It think will be safer here to be specific about what we are looking for.

            sort: 'desc',
            status: 'success',
  

  WDYT?