Update zeppelin REST API - make zeppelin rest-api more conventional

Redefine the concept of rest api to zeppelin:

* [ ] Refactor existing endpoints
* [x] Add root endpoint
* [x] Switch rest url `cxf/zeppelin` to `/api`
* [x] Test rest api
This is nicy but may be we should do something useful here from scratch, i.e as elasticsearch does - display at least current version\build number