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 a good change, though is not complete: ```sessionId``` should be changed to ```jobId``` everywhere too