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. 
Why not just have two methods `int getNamespace(String name)` and `int listAllNamespaces()`? Then there's no need to deal with null, and it's easier to understand IMO.