Nova used to emit versioned and unversioned notiifcations
by default but that changed in https://review.opendev.org/603079/
so now nova emits only unversioned notifications by default.
Watcher listens for versioned notifications so we need to configure
nova to emit both versioned (for Watcher) and unversioned
(for Ceilometer) notifications explicitly.
This adds an override-defaults file so devstack will load up
the nova devstack variable to set the notification_format before
importing and stacking the nova lib script.
Note that this only fixes the non-grenade CI jobs since grenade
requires separate handling for overriding defaults which is proving
hard to do and will be addressed in a separate change.
Partial-Bug: #1831917
Change-Id: I7e441608b38338eecd80e663ed3abe66a89e504f
966a4dfa5f