'Manages an existing snapshot.
Renames the snapshot name to match the expected name for the snapshot.
Error checking done by manage_existing_get_size is not repeated.
:param snapshot:
snapshot ref info to be set
:param existing_ref:
existing_ref is a dictionary of the form:
{\'source-name\': <name of rbd snapshot>}'

def manage_existing_snapshot(self, snapshot, existing_ref):

    if (not isinstance(existing_ref, dict)):
        existing_ref = {'source-name': existing_ref}
    volume_name = utils.convert_str(snapshot.volume_name)
    with RBDVolumeProxy(self, volume_name) as volume:
        snapshot_name = existing_ref['source-name']
        volume.rename_snap(utils.convert_str(snapshot_name), utils.convert_str(snapshot.name))
