Check if advanced image flavor already exists

Flavor added in 79c679683dd93c1df412f81533af6bf72e2395d0 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
1 file changed