Check if advanced image flavor already exists

Flavor added in 79c679683d fails in
at least stable/wallaby neutron-ovn-tempest-slow job, as the flavor
already exists.

This checks if a flavor with this name already exists before trying to
create it

Change-Id: I624f53e39c4bcdd988900aca375ee5e4ddd445fe
Related-Bug: #1940243

devstack/functions.sh

@@ -93,5 +93,7 @@ function create_flavor_for_advance_image {
     local disk=$3
     local vcpus=$4
 
-    openstack flavor create --ram $ram --disk $disk --vcpus $vcpus $name
+    if [[ -z $(openstack flavor list | grep $name) ]]; then
+        openstack flavor create --ram $ram --disk $disk --vcpus $vcpus $name
+    fi
 }