Modify EMR add_jobflow_steps to return a JobFlowStepList with a list of step ids, part of the latest EMR API updates.
Add a basic test for new add_jobflow_steps behavior.
Test a second item to make sure the list is properly processed and not just the first item is returned.
Merge pull request #1838 from danielgtaylor/jobflow-steps

Modify EMR add_jobflow_steps to return a JobFlowStepList. Fixes #1838.