Allow same "label" in one "servers"

Support like a following server_list.yaml:

```yaml
--- #web
servers:
  - config: web-kvm.yaml
    label: foo
    hosts:
      - 10.0.0.1 sv0001
      - 10.0.0.2 sv0001
  - config: web-baremetal.yaml
    label: foo
    hosts:
      - 10.0.0.3 sv0003
```