Verify the attributes of 'set-server-metadata' API

This patch verifies the response attributes of 'set-server-metadata'.
The response attributes of v2 and v3 are the same:
{
    "metadata": {
        "foo": "Foo Value"
    }
}
Partially implements blueprint nova-api-attribute-test

Change-Id: Ifffa537a228819326f00fd0c98271882fb42f8c3
3 files changed