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
- [x] Testing on a distributed cluster. 
I just checked. The ``list()`` methods in ``Store`` never return ``null``. If not found, its always an empty list. This check is redundant. I'll remove it.