Fix for ClusterRoleBinding without a 'subjects' field

ClusterRoleBindings should have the field `subjects`. 
But it is possible to create ClusterRoleBinding wihtout `subjects` field such as the default ClusterRoleBinding `system:node` from kubeadm installlation or by Minikube.
The code didn't verified it the `subjects` field exist and therefore throw an exception.
The fix now verify the this field exist before checking it.

Author: g3rzi

api/api_client_temp.py

@@ -648,7 +648,7 @@ def list_cluster_role_binding(self):
              role_ref = V1RoleRef(api_group=i['roleRef']['apiGroup'], name=i['roleRef']['name'], kind=i['roleRef']['kind'])
              subjects = []
 
-             if i['subjects'] is not None:
+             if 'subjects' in i and i['subjects'] is not None:
                  for s in i['subjects']:
                        namespace = None
                        if 'namespace' in s.keys():