The upstream [Nova upgrade notes][2] suggest that:
After fully upgrading to kilo (i.e. all nodes are running kilo code), you should start a background migration of flavor information from its old home to its new home. Kilo conductor nodes will do this on the fly when necessary, but the rest of the idle data needs to be migrated in the the background. This is critical to complete before the Liberty release, where support for the old location will be dropped. Use "nova-manage db migrate_flavor_data" to perform this transition.
You should run this command as the nova user:
runuser -u nova -- nova-manage db migrate_flavor_data