Entity list truncation by drivers

Keystone supports the ability for a deployment to restrict the number of entries returned from list operations, typically to prevent poorly formed searches (e.g. without sufficient filters) from becoming a performance issue.

These limits are set in the configuration file, either for a specific driver or across all drivers. A global list_limit set in [DEFAULT] section of keystone is considered in case no limit is set for specific driver. These limits are read at the Manager level and passed into individual drivers as part of the Hints list object. A driver should try and honor any such limit if possible, but if it is unable to do so then it may ignore it (and the truncation of the returned list of entities will happen at the API level by wrap_collection method).