XD-2030 Fix job instances/step GET endpoints

 - Set producible media type for job instances/step GET
endpoints
  - /jobs/instances/{instanceId}
  - /jobs/instances?jobname={jobName}
  - /jobs/executions/{jobExecutionId}/steps
Update description of how to override rabbit settings in modules.yml

- sync ./spring-xd-yarn/site/config/modules.yml and ./config/modules/modules.yml