mgr/orchestrator: get_hosts return `HostSpec` instead of `InventoryDevice`

mgr/orchestrator: get_hosts return `HostSpec` instead of `InventoryDevice`

This fixes a bad symmetry problem:

previsously:
```python
def add_host(host: HostSpec): ...
def get_hosts() -> List[InventoryDevice]: ...
```

now:
```python
def add_host(host: HostSpec): ...
def get_hosts() -> List[HostSpec]: ...
```

Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>