[REVIEW] NXCM-4884: Reworked Kenai realm according to AuthAPI

Kenai Realm made to conform the AuthAPI of Kenai. That means GET changed to POST with urlencoded form.

Wiki:
https://kenai.com/projects/kenaiapis/pages/AuthAPI

Now it uses the form:
```
POST /api/login/authenticate.json
Content-Type: application/x-www-form-urlencoded

username={user}&password={pass}
```

The mock KenaiMockAuthcServlet also adapted.