Add NOVA_NOTIFICATION_FORMAT variable
Add NOVA_NOTIFICATION_FORMAT variable

Nova change https://review.opendev.org/603079/ changed the
default configuration to send only unversioned notfications rather
than both versioned and unversioned notifications. This could break
unsuspecting downstream projects (like Watcher) whose CI jobs are
not explicitly configuring nova for the types of notifications
they need but are just relying on getting both per the previous
default of the config option.

This adds a variable which defaults to "unversioned" to match the
nova default but allows downstream CI jobs to easily configure
another value.

Needed by https://review.opendev.org/663332/

Change-Id: Ied9d50b07c368d5c2be658c744f340a8d1ee41e0

dtroyer/devstack,nawawi/openstack,nawawi/openstack,dtroyer/devstack,openstack-dev/devstack,openstack-dev/devstack