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
Why here and below verbs are part of URLs? Also AFAIK there is single job tree right now, so no need for s at the end.
I suggest we keep @GET /jobTree and @POST /jobTree