The upgrading flag introduced in https://git.opendaylight.org/gerrit/#/c/65299/ must be able to be set pre boot by a file which director can set.

As is this currently is not possible, and probably requires making it use injecting the config via odl:clustered-app-config, like the TracingBroker does.

While we're at this we'll also change the YANG model introduced in c/65299 to match the suggestions mades by shague post merge of that initial change.

One this is implemented (and merged), we need to create a task in LaunchPad for director to create this file during upgrade (not update) before triggering the full sync, and then reset its value via REST Conf call when done, and remove that file again.

Alternatively, perhaps Director could just set this flag via REST Conf immediately after ODL has come up but before letting any traffic hit it?

We should start working on this task once there is at least 1 usage of UpgradeState; as of right now we have the infra (from c/65299) but no users of it, yet.