Verify 'update-server-metadata' APIs attributes

This patch verifies the response attributes of 'update-server-metadata'.

The response attributes of v2 and v3 are the same:
{
    "metadata": {
        "foo": "Foo Value"
    }
}

Response code for V3 API is 201.

Partially implements blueprint nova-api-attribute-test

Change-Id: If70c4cd31ed513276d0a714edc06d4878913ec48
Merge "Verify 'update-server-metadata' APIs attributes"