CDAP-772 Feature/cdap 772 namespace service

Namespaces APIs to manage namespaces metadata.
Question: 
Should we create and use ``Id.Namespace`` instead of using namespace name as ``Key``?

- [x] Namespace service to create, list, delete namespaces
- [x] Unit testing, standalone testing
- [ ] Testing on a distributed cluster. 
Shouldn't be warn. Something not found on GET request is expected. Logged as warn, which most of time will get logged (which becomes a point for DOS). It should be debug or even trace.