responseElements["status"] is not always a string

[et304383]

This is in some of my CloudTrail records:

      "responseElements": {
        "status": {
          "state": "SUCCEEDED"
        },

I think com.graylog2.input.cloudtrail.json.CloudTrailResponseElements needs to be fixed to not expect status is only ever a string.

[joschi]

Refs #6