steps subdirs on EMR are now step IDs

Noticed this while fixing #888.

When we fetch logs, we currently look inside steps/<step_num>/. But EMR now uses step IDs (which look like s-AAAAAAAA), so we need to first look up the mapping between step num and step ID with DescribeSteps, and then look in the corresponding directory (steps/s-AAAAAAAA/).

This is true both on S3 and via SSH, and true on both 2.x and 3.x AMIs.