Verify list extensions attributes of V2/V3 APIs

This patch adds the JSON schema for Nova V2/V3 list extensions APIs
includes the attributes to block the backward incompatibility change
in the future.

The response body of v2 is the below:
{
    "extensions": [
        {
            "updated": "2011-06-09T00:00:00+00:00",
            "name": "Multinic",
            "links": [],
            "namespace": "http://xxx.xxx.xxx/xxx/xxx",
            "alias": "NMN",
            "description": "Multiple network support."
        }
    ]
}

The response body of v3 is the below:
{
    "extensions": [
        {
            "name": "Multinic",
            "alias": "NMN",
            "description": "Multiple network support.",
            "version": 1
        }
    ]
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I0597a7b0d0732b00b77e4a6f0806391054470a6b
Merge "Verify list extensions attributes of V2/V3 APIs"