Use oslo-versioned-objects to help with dealing with upgrades

https://etherpad.openstack.org/p/kilo-crossproject-upgrades-and-versioning

This blueprint improves the way Keystone deals with versioning of objects (of all sorts db/rpc/rest/templates/plugins).

Nova has come up with the idea of versioned objects, that Ironic has also now used. This has been proposed as an oslo library https://review.openstack.org/#/c/127532/ which has been already implemented and available at https://github.com/openstack/oslo.versionedobjects

There is a work being done in Heat and Cinder which may be a good starting point for Keystone (ideas, reliability):
https://blueprints.launchpad.net/heat/+spec/versioned-objects
https://blueprints.launchpad.net/cinder/+spec/cinder-objects

The main idea of versioned objects is to create a wrapper objects that would be an abstraction layer above the currently used DB objects. Versioned-objects will help us deal with DB schema being at a different version than the code expects. This will allow Keystone to be operated safely during upgrades.