(EMR) Fix bug where StepStatus was parsed as ClusterStatus, breaking step timelines

This bug caused step timelines to lack the startdatetime attribute. (Cluster timelines have a different attribute; readydatetime).

Relevant docs:
http://docs.aws.amazon.com/ElasticMapReduce/latest/API/API_DescribeStep.html
http://docs.aws.amazon.com/ElasticMapReduce/latest/API/API_ListSteps.html
http://docs.aws.amazon.com/ElasticMapReduce/latest/API/API_StepStatus.html