Validate image metadata attributes of Nova APIs

This patch adds the JSON Schema for Nova V2 Image metadata APIs
(list, set, update) and validate the response with added JSON
Schema to block the backward incompatibility change in the future.

Response body of above APIs is below-

{
    "metadata":
    {

    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I923dc1f8e5df96339b14221a68086039c36e22dc
Check attributes of image meta item Nova APIs

This patch adds the JSON Schema for Nova V2 Image meta item APIs
(set & get meta item) and validate the response with added JSON
Schema to block the backward incompatibility change in the future.

Response body of above APIs is below-

{
    "meta":
    {

    }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: Ic00fc3b4750057b641ef01738e34f7242e27bb1d
Merge "Validate image metadata attributes of Nova APIs"
Merge "Check attributes of image meta item Nova APIs"