'Manages an existing LUN in the array.
The LUN should be in a manageable pool backend, otherwise return error.
Rename the backend storage object so that it matches the
`volume[\'name\']` which is how drivers traditionally map between a
cinder volume and the associated backend storage object.
LUN ID or name are supported in `existing_ref`, like:
.. code-block:: none
existing_ref:{
\'source-id\':<LUN id in Unity>
or
.. code-block:: none
existing_ref:{
\'source-name\':<LUN name in Unity>'

def manage_existing(self, volume, existing_ref):

    lun = self._get_referenced_lun(existing_ref)
    lun.modify(name=volume.name)
    return {'provider_location': self._build_provider_location(lun_id=lun.get_id(), lun_type='lun'), 'provider_id': lun.get_id()}
