Simplify volume usage with containerop (kubeflow#783)

change function name to mount_pvc

sdk/python/kfp/onprem.py

@@ -0,0 +1,22 @@
+
+def mount_pvc(pvc_name='pipeline-claim', volume_name='pipeline', volume_mount_path='/mnt/pipeline'):
+    """
+        Modifier function to apply to a Container Op to simplify volume, volume mount addition and 
+        enable better reuse of volumes, volume claims across container ops.
+        Usage:
+            train = train_op(...)
+            train.apply(mount_pvc('claim-name', 'pipeline', '/mnt/pipeline'))
+    """
+    def _mount_pvc(task):
+        from kubernetes import client as k8s_client
+        local_pvc = k8s_client.V1PersistentVolumeClaimVolumeSource(claim_name=pvc_name)
+        return (
+            task
+                .add_volume(
+                    k8s_client.V1Volume(name=volume_name, persistent_volume_claim=local_pvc)
+                )
+                .add_volume_mount(
+                    k8s_client.V1VolumeMount(mount_path=volume_mount_path, name=volume_name)
+                )
+        )
+    return _mount_pvc