Check add/remove flavor access APIs attributes

This patch adds the JSON schema for Nova V2/V3 add & remove flavor
access APIs response and validate the response with added JSON schema to
block the backward incompatibility change in the future.

The response body of V2 & V3 for add & remove flavor access APIs are same
and given below:

{
    "flavor_access": [
        {
            "flavor_id": "10",
            "tenant_id": "fake_tenant"
        }
}

Partially implements blueprint nova-api-attribute-test

Change-Id: I0e876494b9d1bda418076844ed3211f5513af76c
Add unit tests for commands

This commit adds commands function.

Partially implements bp unit-tests

Change-Id: If5e037546965e338ebb9a171a178826b4e3136d6
Merge "Add unit tests for commands"
Merge "Check add/remove flavor access APIs attributes"