Return user's roles in response to /tenants/tenantId/users

Currently, user's roles in given tenant are not returned in response to /tenants/tenantId/users.
So, for example, if we need to get admins or user of a given tenant, we should:
1) get user list for tenant;
2) for each user get his roles (an HTTP query /users/userId/roleRefs _for each user_) and parse the responses.

A better solution is to return roles in a single query:
{
    "users": {
        "links": [
            {
                "href": "http://127.0.0.1:5001/v2.0/tenants/ANOTHER:TENANT/users?'marker=34&limit=10'",
                "rel": "next"
            }
        ],
        "values": [
            {
                "email": "abc9",
                "enabled": true,
                "id": "admin",
                "tenantId": "ANOTHER:TENANT",
                "tenantRoles": [
                    "Admin",
                    "user"
                ]
            },
            {
                "email": "aa",
                "enabled": true,
                "id": "joeadmin",
                "tenantId": "ANOTHER:TENANT",
                "tenantRoles": [
                    "user"
                ]
            }
        ]
    }
}

Here is a proposed patch:
https://github.com/aababilov/keystone/commit/7f37fdac4a71ceecb8ce8fae6b56b8321a0f2f72