title: Mounting volume issue through KubernetesPodOperator in GKE airflow
body: <p>My DAG is getting stuck in running state with a log message "had an event of type Pending" in stack driver. My DAG works fine without volume mount but looks like there is an issue with volume mount. Can you please someone help here?</p>
  
  <p>I am trying to mount "/data/storage" path to "/storage" location in container.</p>
  
  <pre><code>from airflow import DAG
  from airflow.contrib.operators.kubernetes_pod_operator import KubernetesPodOperator
  from airflow.contrib.kubernetes.volume import Volume
  from airflow.contrib.kubernetes.volume_mount import VolumeMount
  from datetime import timedelta
  default_args = {
                      'owner': '',
                      'start_date': '2020-03-31 03:40:34',
                      'depend_on_past': False,
                      'email_on_failure': False,
                      'email_on_retry': False,
                      'retries': 0,
                      'retry_delay': timedelta(minutes=5)
                      } 
  volume_mount = VolumeMount('test-volume',
                              mount_path='/data/storage',
                              sub_path='storage',
                              read_only=True)
  volume_config= {
      'persistentVolumeClaim':
        {
          'claimName': 'test-volume'
        }
      }
  volume = Volume(name='test-volume', configs=volume_config)
  dag = DAG('dag-name', default_args=default_args, schedule_interval=None, catchup=False, concurrency=1, max_active_runs=1)
  task1 = KubernetesPodOperator(
                                      # The ID specified for the task.
                                      task_id='task1',
                                      # Name of task you want to run, used to generate Pod ID.
                                      name='task1',
                                      namespace='default',
                                      env_vars={'var1': 'value1'},
                                      image='someimage',
                                      volumes=[volume],
                                      volume_mounts=[volume_mount],
                                      arguments=['arg.json'],
                                      startup_timeout_seconds=3600,
                                      image_pull_policy='Always',
                                      dag=dag)
  
  </code></pre>