Fix emr endpoints for eu-central-1 and cn-north-1

Added integration tests as an additional safety net here.
Merge branch 'emr-endpoint-fix-2' into develop

* emr-endpoint-fix-2:
  Fix emr endpoints for eu-central-1 and cn-north-1
Parse empty xml nodes that are strings are an empty string

This allows `get_*` operations to better round trip with the
corresponding `put_*` operations.