Support extra field in project API.

[junoyoon]

Currently openstack's identityV3 api supports extra field on project and user.
The extra field can be sent to keystone with as top level json element and keystone send back the extra field as top level json element.

• For example, when calling project update API like followings.

{
    "project": {
        "is_domain": true,
        "description": "My updated project",
        "links": {
            "self": "http://localhost:5000/v3/projects/93ebbcc35335488b96ff9cd7d18cbb2e"
        },
        "enabled": true,
        "id": "93ebbcc35335488b96ff9cd7d18cbb2e",
        "parent_id": null,
        "domain_id": "default",
        "name": "myUpdatedProject"
        "my_extra_key": "wow" <== HERE
    }
}

Keystone sends back extra fields as the top level json element.

{
    "project": {
        "is_domain": true,
        "description": "My updated project",
        "links": {
            "self": "http://localhost:5000/v3/projects/93ebbcc35335488b96ff9cd7d18cbb2e"
        },
        "is_domain": true
        "enabled": true,
        "id": "93ebbcc35335488b96ff9cd7d18cbb2e",
        "parent_id": null,
        "domain_id": "default",
        "name": "myUpdatedProject",
        "my_extra_key": "wow"   <== HERE
    }
}

Currently openstack4j doesn't support extra field yet. However, the extra field is tje very important feature when developers make a system based on keystone and want to put some meta data on project.