Adds--max-failures flag tocoder exp scaletest create-workspaces so that we can tolerate a few failures without failing the command.

When running our scale test infra, we create Kubernetes Jobs to create the initial cluster workspaces, then we have load-generation jobs that depend on them. At high scale, it's kind of expected that some of the requests will fail: even with 99.9% success, you still expect one failure per 1000. It's useful to be able to carry on with the scale test anyway and proceed to traffic generation.