Make cluster_context templated in KubernetesPodOperator

[mati-o]

---

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