Nova changed the default notification_format from "both" to
"unversioned" in Train [1]. Without configuring nova in the
grenade job we are not testing the nova versioned notification
handler code during upgrades.
Note that grenade only runs stack.sh on the base (old) side so
this change has to depend on a devstack stable/stein change to
add the NOVA_NOTIFICATION_FORMAT variable that we override.
Closes-Bug: #1831917
Depends-On: Ied9d50b07c368d5c2be658c744f340a8d1ee41e0
[1] https://review.opendev.org/603079/
Change-Id: I94c2d14477da185310e0fec596a1ad6436b802f1
9c9f336f10