Make cluster_context templated in KubernetesPodOperator

[mati-o]

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Following Airflow's best practice guide for Variables, I have tried replacing all of my variables' usage in KubernetesPodOperator, such as image, config_file etc.. to use the templated syntax. However, for cluster_context this didn't work since it's not templated, as in the example below:

default_args = {
- 'cluster_context': Variable.get('k8s_cluster_name')+ 'cluster_context': '{{ var.value.k8s_cluster_name }}' 
}

I'd love to have this small feature incorporated, let me know of anything required beyond this commit