Upgrade controllers with no API downtime for neutron

Currently pretty much every major upgrade requires full shutdown for all neutron-server instances for the time while upgrade process is running. The downtime is due to the need to run alembic scripts that modify schema and transform data. Neutron-server instances are currently not resilient to working with older sch...

Adopt oslo.versionedobjects for database interactions for neutron

This blueprint will give neutron a proper object facade to hide interactions with database behind. With that, we will have a cleaner code base as well as potential to implement support for no-downtime upgrades for controllers.