AWS::EMR::Cluster - StepConcurrencyLevel

## 1. Title

#### AWS::EMR::Cluster-StepConcurrencyLevel

## 2. Scope of request

The EMR::Cluster resource should support setting the StepConcurrencyLevel attribute.

## 3. Expected behavior

This should modify the concurrency value for a cluster, enabling multiple Steps to run simultaneously.

It appears you can set this value on an existing cluster (view [API call](It appears you can set this value on an existing cluster, so it'd be cool to )), so it'd be cool if modifying this value could avoid a cluster Replacement.

## 4. Suggest specific test cases

Besides avoiding cluster Replacement, if possible, it appears this feature is only supported on EMR releases 5.28.0 and above. So accounting for that behavior would be useful.

## 5. Helpful Links to speed up research and evaluation

[Feature release blog post](https://aws.amazon.com/about-aws/whats-new/2019/11/amazon-emr-now-allows-you-to-run-multiple-steps-in-parallel-cancel-running-steps-and-integrate-with-aws-step-functions/)

## 6. Category (required) - Will help with tagging and be easier to find by other users to +1

7. Analytics (Athena, EMR, Glue,...)

## 7. Any additional context (optional)

_none_