'Brings an existing backend storage object under Cinder management.
existing_ref is passed straight through from the API request\'s
manage_existing_ref value, and it is up to the driver how this should
be interpreted.  It should be sufficient to identify a storage object
that the driver should somehow associate with the newly-created cinder
snapshot structure.
There are two ways to do this:
1. Rename the backend storage object so that it matches the
snapshot[\'name\'] which is how drivers traditionally map between a
cinder snapshot and the associated backend storage object.
2. Place some metadata on the snapshot, or somewhere in the backend,
that allows other driver requests (e.g. delete) to locate the
backend storage object when required.
If the existing_ref doesn\'t make sense, or doesn\'t refer to an existing
backend storage object, raise a ManageExistingInvalidReference
exception.
:param snapshot:     Cinder volume snapshot to manage
:param existing_ref: Driver-specific information used to identify a
volume snapshot'

def manage_existing_snapshot(self, snapshot, existing_ref):

    return
