Boskos does not allow duplicate resource name even for different types

*Originally filed as https://github.com/kubernetes/test-infra/issues/15054 by @amwat*

```yaml
resources:
- type: bar
  names: 
  - foo
- type: qwe
  names:
  - foo
```

results in `{"error":"duplicated resource name: foo","level":"fatal","msg":"failed to create ranch!" }`

This seems like something we should support?

/cc @krzyzacy @ixdy @sebastienvas 
