nixosTests.flannel: port to python, unbreak
nixosTests.flannel: port to python, unbreak

For reasons yet unknown, the vxlan backend doesn't work (at least inside
the qemu networking), so this is moved to the udp backend.

Note changing the backend apparently also changes the interface name,
it's now `flannel0`, not `flannel.1`

fixes #74941

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs