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

We are looking to improve the way we deal with versioning (of all sorts db/rpc/rest/templates/plugins). Nova has come up with the idea of versioned objects, that Ironic has also now used. There is an oslo library, which is already used in Heat.

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

Versioned-objects will help us deal with DB schema being at a different version than the code expects. This will allow Barbican to be operated safely during upgrades.

Looking forward as we pass more and more data over RPC we can make use of versioned-objects to ensure upgrades happen without spreading the version dependant code across the code base.