EmrStepSensor to auto detect stepid's

[pedresnyman]

Description

Currently the EmrStepSensor only looks at one step at a time. Enhancing the EmrStepSensor to retrieve all steps on a specified cluster if no stepId was provided.

Use case / motivation

If no step_id was provided the EmrStepSensor wil retrieve all steps on a specified cluster.
This allows you to retrieve all steps and their states with a singe api call.

What happened

We ran into a throttle limit.

botocore.exceptions.ClientError: An error occurred (ThrottlingException) when calling the DescribeStep operation (reached max retries: 4): Rate exceeded

With the enhancement of the EmrStepSensor that is taken care of and also then the EmrStepSensor is more dynamic.

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