Add execution role parameter to AddStepsOperator

[syedahsn]

EMR launched a new feature which allows configuring runtime roles for EMR steps. This PR allows users to use this feature with the EmrAddStepsOperator. System test has been updated to use this feature.

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[uranusjr]

Needs to resolve conflicts.

[syedahsn]

Made a minor change with how the new parameter gets used in the hook. Because we are using @mock_emr decorator in some of the tests, it applies boto3 validation rules on the parameters passed, which include no None values, as well as a minimum length (otherwise I could have done a simpler ExecutionRoleArn=execution_role_arn or "").

[Taragolis]

Made a minor change with how the new parameter gets used in the hook. Because we are using @mock_emr decorator in some of the tests, it applies boto3 validation rules on the parameters passed, which include no None values, as well as a minimum length (otherwise I could have done a simpler ExecutionRoleArn=execution_role_arn or "").

I think this originally comes from botocore service definition, example for input of AddJobFlowStepsInput.

I've also found that if set some of optional parameters set to None or empty string then botocore do not propagate it to API call but sometimes it raise an error ¯\_(ツ)_/¯