Support multiple kCTF clusters in the same project

This is desired if you want to provide challenges in different regions.

Some things that are broken currently:
- [x] subnet is hardcoded in the startup script
- [x] DNS entries get overwritten when deploying challenges with the same name
- [x] With multiple clusters I am not sure if I should be using a single kctf-conf/ directory with different cluster configs in there, or if it's ok to have multiple kctf-conf directories
- [x] Given that the cluster configs are setup using symlinks I cannot run commands in multiple clusters in different shells. Maybe we should allow changing the cluster config based on an environment variable.
- [ ] https://github.com/google/kctf/issues/199 two tasks with the same name override each other