[AIRFLOW-5546] Fix DataprocClusterCreateOperator

[turbaszek]

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  This PR improves changes proposed in AIRFLOW-3149 Support dataproc cluster deletion on ERROR #4064. Returning True was raising an KeyError when I was running system test.

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[XD-DENG]

I have never looked into this operator previously, but one point that I would like to point out:
Earlier this method is return True/False, and after your change it will return mixed things (False/result from DataProcHook.find_cluster(), which can be an actual value or None). I do not find this a good practice.

[turbaszek]

I agree. I did this because it's a restricted method used only in execute where I handle this output. And that's better than failing operator (system tested).

Probably I will close this issue soon because I am refactoring Dataproc hook / operators.

[potiuk]

I agree it's strange to do it this way. It should be consistently one type. Waiting for the refactor :)

[turbaszek]

Addressed in #6371