[AIRFLOW-5661] Fix create_cluster method of GKEClusterHook

[turbaszek]

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Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title. For example, "[AIRFLOW-XXX] My Airflow PR"
  • https://issues.apache.org/jira/browse/AIRFLOW-5661

Description

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  This PR fixes create_cluster method of GKEClusterHook. The method uses get_cluster which already returns a string, so calling self_link once more rises error.

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[turbaszek]

@mik-laj can you tak a look?

[mik-laj]

Everything works as expected