Verify the response attributes of 'list_hosts'

This patch verifies the response attributes of list_hosts.
The response body of v2 and v3 API are in the same format:

"hosts": [
    {
        "host_name": "b6e4adbc193d428ea923899d07fb001e",
        "service": "conductor",
        "zone": "internal"
    }
]

Partially implements blueprint nova-api-attribute-test

Change-Id: Ia26d14e6fcec0e8f76224669ebde57ff8840605c
Merge "Verify the response attributes of 'list_hosts'"