Verify the response attributes of 'shutdown/reboot_host' API

This patch verifies the response attributes of 'shutdown/reboot_host' API.
The response body format of v2:
{
    "host": "d2576862a2144ee6ad37d9e1938460a2",
    "power_action": "shutdown/reboot"
}
The response body format of v3:
{
    "host": {
        "host": "77cfa0002e4d45fe97f185968111b27b",
        "power_action": "shutdown/reboot"
    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I2e1ede52f7461e9d6fa583a00afc261f77314908
Merge "Verify the response attributes of 'shutdown/reboot_host' API"