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. 
POST would mean that you can post the same namespace twice, and it would create two namespaces of the same name. I think PUT is the correct semantic here. Whether you support "update" is a separate question.