'Return an available entry to store a record.'

def _get_available_record_entry(self, zone):

    entries = zone.extra
    for entry in range(1, (MAX_RECORD_ENTRIES + 1)):
        subdomain = entries.get(('S%s' % entry))
        _type = entries.get(('T%s' % entry))
        data = entries.get(('D%s' % entry))
        if (not any([subdomain, _type, data])):
            return entry
    return None
