def _extract_attached_to(disk):
    users = disk.get('users', [])
    if not users:
        return None
    if len(users) > 1:
        raise GCEVolumeException(
            "Volume is attached to more than one instance:{}".format(disk))
    return unicode(users[0].split('/')[-1])
Given a GCE disk resource, determines the unicode name of the
dataset/ETHPy150Open ClusterHQ/flocker/flocker/node/agents/gce.py/_extract_attached_to